The Power of Movement in Plants

The same remarkable experiment was next tried, as in the case of the bean; namely, little squares. of exactly the same size of the card-like sanded paper and of very thin paper (the thicknesses of which have been given under Vicia faba) were attached with shellac on opposite sides (as accurately as could be done) of the tips of 13 radicles, suspended in damp air, at a temperature of 65°-66° F. The result was striking, for 9 out of these 13 radicles became plainly, and 1 very slightly, curved from the thick paper towards the side bearing the thin paper. In two of these cases the apex became completely hooked after two days; in four cases the deflection from the per- pendicular and from the side bearing the thick paper, amounted in from two to four days to angles of 90°, 72°, 60°, and 49°, but in two other cases to only 18° and 15°. It should, however, be stated that in the

ease in which the deflection was 49°, the two squares had accidentally come into contact on one side of the apex, and thus formed a lateral gable; and the deflec- tion was directed in part from this gable and in part from the thick paper. In three cases alone the radicles were not affected by the difference in thickness of the squares of paper attached to their tips, and conse- quently did not bend away from the side bearing the stiffer paper. Zea mays: Sensitiveness of the apex of the Radicle to contact.—A large number of trials were made on this plant, as it was the only monocotyledon on which we experimented. An abstract of the results will suffice. In the first place, 22 germinating seeds were pinned to cork-lids without any object being attached to their radicles, some being exposed to a temperature of 65°~ 66° F., and others to between 74° and 79°; and none of them became curved, though some were a little inclined to one side. A few were selected, which from having germinated on sand were crooked, but when suspended in damp air the terminal part grew straight down- wards. This fact having been ascertained, little squares of the card-like paper were affixed with shellac, on several occasions, to the tips of 68 radicles. Of these the terminal growing part of 39 became within 24 h. conspicuously curved away from the attached squares and from the perpendicular; 13 out of the 39 forming hooks with their points directed towards the zenith, and 8 forming loops. Moreover, 7 other radicles out of the 68, were slightly and two doubtfully deflected from the cards. There remain 20 which were not affected; but 10 of these ought not to be counted; for one was diseased, two had their tips quite sur rounded by shellac, and the squares on 7 had slipped so as to stand parallel to the apex, instead of obliquely

on it. There were therefore only 10 out of the 68 which certainly were not acted on. Some of the radicles which were experimented on were young and short, most of them of moderate length, and two or three exceeded three inches in length. ‘he curva- ture in the above cases occurred within 24 h., but it was often conspicuous within a much shorter period. For instance, the terminal growing part of one radicle was bent upwards into a rectangle in 8 h. 15 m., and of another in 9h. On one occasion a hook was formed in 9 h. Six of the radicles in a jar containing nine seeds, which stood on a sand-bath, raised to a temperature varying from 76° to 82° F., became hooked, and a‘seventh formed a complete loop, when first looked at after 15 hours.

The accompanying figures of fuur germinating seeds (Fig. 69) show, firstly, a radicle (A) the apex of which has become so much bent away from the attached square as to form a hook. Secondly (B), a hook converted through the continued irritation of the card, aided perhaps by geotropism, into an almost complete circle or loop, The tip in the act of forming a loop generally rubs against the upper part of the radicle, and pushes off the attached square; the loop then contracts or closes, but never disappears; and the apex afterwards grows vertically downwards, being no longer irritated by any attached object. This frequently occurred, and is represented at C. The jar above mentioned with the six hooked radicles and another jar were kept for two additional days, for the sake of observing how the hooks would be modified. Most of them became converted into simple loops, like that figured at C; but in one case the apex did not rub against the upper part of the radicle and thus remove the card; and it consequently made, owing

to the continued irritation from the card, two complete loops, that is, a helix of two spires; which afterwards became pressed closely together. Then geotropism prevailed and caused the apex to grow perpendicularly downwards. Jn another case, shown at (D), the apex Cc D. Zea mays: radicles excited to bend away from the little squares of card attached to one side of their tips. in making a second turn or spire, passed through the first loop, which -was at first widely open, and in doing so knocked off the card; it then grew perpen- dicularly downwards, and thus tied itself into a knot, which soon became tight!

Secondary Radicles of Zea.—A short time after the first radicle has appeared, others protrude from the seed, but not laterally from the primary one. Ten of these secondary radicles, which were directed obliquely downwards, were experimented on with very small squares of card attached with shellac to the lower sides of their tips. If therefore the squares acted, the radicles would bend upwards in opposition to gravity. The jar stood (protected from light) on a sand-bath, which varied between 76° and 82° F. After only 5 h. one appeared to be a little deflected from the square, and aftcr 20 h. formed a loop. Four others were considerably curved from the squares after 20 h., and three of them became hooked, with their tips pointing to the zenith,—one after 29 h. and the two others after 44 h. By this latter time a sixth radicle had become bent at a right angle from the side bearing the square. Thus altogether six out of the ten secondary radicles were acted on, four not being affected. There can, therefore, be no doubt that the tips of these secondary radicles are sensitive to slight contact, and that when thus excited they cause the upper part to bend from the touching object; but generally, as it appears, not in so short a time as in the case of the first-formed radicle.

Sachs made the interesting discovery, a few years ago, that the radicles of many seedling plants bend towards an adjoining damp surface.* We shall here endeavour to show that this peculiar form of sensitive- ness resides in their tips. The movement is directly the reverse of that excited by the irritants hitherto considered, which cause the growing part of the radicle to bend away from the source of irritatiou. In our experiments we followed Sachs’ plan, and sieves with seeds germinating in damp sawdust were sus- pended so that the bottom was generally inclined at 40° with the horizon. If the radicles had been acted on solely by geotropism, they would have grown out of the bottom of the sieve perpendicularly down- wards; but as they were attracted by the adjoining damp surface they bent towards it and were deflected 50° from the perpendicular. For the sake of ascertain- ing whether the tip or the whole growing part of the radicle was sensitive to the moist air, a length of from 1 to 2 mm. was coated in a certain number of cases with a mixture of olive-oil and lamp-black. This mixture was made in order to give consistence to the oil, so that a thick layer could be applied, which would exclude, at least to a large extent, the moist air, and would be easily visible. A greater number of experiments than those which were actually tried would have been necessary, had not it been clearly established that the tip of the radicle is the part which is sensitive to various other irritants.

Phaseolus multiflorus.—Twenty-nine radicles, to which no- thing had been done, growing out of a sieve, were observed at the same time with those which had their tips greased, and for an equal length of time. Of the 29, 24 curved them- selves so as to come into close contact with the bottom of the sieve. The place of chief curvature was generally at a distance of 5 or 6 mm. from the apex. Eight radicles had their tips greased for a length of 2 mm., and two others for a length of 13 mm.; they were kept at a temperature of 15°-16°C. After intervals of from 19 h. to 24 bh. all were still vertically or almost vertically dependent, for some of them had moved towards the adjoining damp surface by about 10°. They had therefore not been acted on, or only slightly acted on, by the damper air on one side, although the whole upper part was freely exposed. After 48 h. three of these radicles became

considerably curved towards the sieve ; and the absence of curva- ture. in some of the others might perhaps be accounted for by their not having grown very well. But it should be observed that during the first 19 h. to 24 h. all grew well; two of them having increased 2 and 3 mm. in length in 11 h.; five others increased 5 to 8 mm. in 19 h.; and two, which had been at first 4 and 6 mm. in length, increased in 24 h. to 15 and 20 mm. The tips of 10 radicles, which likewise grew weil, were coated with the grease for a length of only | mm., and now the result was somewhat different; for of these 4 curved themselves to the sieve in from 21 h. to 24 h., whilst 6 did not do so. Five of the latter were observed for an additional day, and now all excepting one became curved to the sieve.

The tips of 5 radicles were cauterised with nitrate of silver, and about 1 mm. in length was thus destroyed. They were observed for periods varying betwecn 11h. and 24 h., and were found to have grown well. One of them had curved until it came into contact with the sieve; another was curving towards it; whilst the remaining three were still vertically dependent. Of 7 not cauterised radicles observed at the same time, all had come into contact with the sieve.

The tips of 11 radicles were protected by moistened gold- beaters’ skin, which adheres closely, for a length varying from 1; to 24 mm. After 22 h. to 24 h,6 of these radicles were clearly bent towards or had come into contact with the sieve; 2 were slightly curved in this direction, and 3 not atall. All had grown well. Of 14 control specimens observed at the same time, all excepting one had closely approached the sieve. It appears from these cases that a cap of goldbeaters’ skin checks, though only to a slight degree, the bending of the radicles to an adjoining damp surface. Whether an extremely thin sheet of this substance when moistened allows moisture from the air to pass through it, we do not know. One case indicated that the caps were sometimes more efficient than appears from the above results; for a radicle, which after 23 h. had only slightly approached the sieve, had its cap (13 mm. in length) removed, and during the next 15 h. it curved itself abruptly towards the source of moisture, the chief seat of curvature being at a distance of 2 to 3 mm. from the apex.

Vicia fubs.—The tips of 18 radicles were coated with the grease for a length of 2 mm.; and it should be remembered that with these radicles the seat of chief curvature is about 4 or 5 mm. from the apex. Four of them were examined after 22 h., three after 26 h., and six after 86 h., and none had been attracted towards the damp lower surface of the sieve. In another trial 7 radicles were similarly treated, and 5 of them still pointed perpendicularly downwards after 11 h., whilst 2 were a little curved towards the sieve; by an accident they were not subsequently observed. In both these trials the radicles grew well; 7 of them, which were at first from 4 to 11 mm. in length, were after 11 h. between 7 and 16 mm.; 3 which were at first from 6 to 8 mm. after 26 h. were 115 to 18 mm. in length; and lastly, 4 radicles which were at first 5 to8 mm. after 46 h. were 18 to 23 mm. in length. The control or ungreased radicles were not invariably attracted towards the bottom of the sieve. But on one occasion 12 out of 13, which were observed for periods between 22 h. and 36 h., were thus attracted. On two other occasions taken together, 38 out of 40 were similarly attracted. On another occasion only 7 out of 14 behaved in this manner, but after two more days the proportion of the curved increased to 17 out of 23. On a last occasion only 11 out of 20 were thus attracted. If we add up these numbers, we find that 78 out of 96 of the control specimens curved themselves towards the bottom of the sieve. Of the specimens with greased tips, 2 alone out of the 20 (but 7 of these were not observed for a sufficiently long time) thus curved themselves. We can, therefore, hardly doubt that the tip for a length of 2 mm. is the part which is sensitive to a moist atmosphere, and causes the upper part to bend towards its source.

The tips of 15 radicles were cauterised with nitrate of silver, and they grew as well as those above described with greased tips. After an interval of 24 h., 9 of them were not at all curved towards the bottom of the sieve; 2 were curved towards it at angles of 20° and 12° from their former vertical position, and 4 had come into close contact with it. Thus the destruc- tion of the tip for a length of about 1 mm. prevented the curva- ture of the greater number of these radicles to the adjoining damp surface. Of 24 control specimens, 23 were bent to the sieve, and on a second occasion 15 out of 16 were similarly curved in a greater or less degree. These control trials are included in those given in the foregoing paragraph.

Avena sativa.—The tips of 13 radicles, which projected between 2 and 4 mm. from the bottom of the sieve. many of them not quite perpendicularly downwards, were cozted with the black grease for a length of from 1 to 1; mm. The sieves were inclined at 3U° with the horizon. The greater number of these radicles were examined after 22 h., and a few after 25 h., and within these intervals they had grown so quickly as to have nearly doubled their lengths. With the ungreased radicles the chief scat of curvature is at a distance of not less than between 35 and 55 mm., and not more than between 7 and 10 mm. from the apex. Out of the 18 radicles with greased tips, 4 had not moved at all towards the sicve; 6 were deflected towards it and from the perpendicular by angles varying between 10° and 35° ; and 3 had come into close contact with it. It appears, therefore, at first sight that greasing the tips of these radicles had checked but little their bending to the adjoining danip surface. But the inspection of the sicves on two occasions produced a widely different impression on the mind; for it was impossible to bdhold the radicles with the black greased tips projecting from the bottom, and all those with ungreased tips, at least 40 to 50 in number, clinging closely to it, and fcc] any doubt that the greasing had produced a great effect. On close examination only a single ungreased radicle could be found which had not become curved towards the sieve. It is probable that if the tips had been protected by grease for a length of 2 mm. instead of from 1 to 1; mm., they would not have been affected by the moist air and none would have become curved.

Triticum vulgare.—Analogous trials were made on 8 radicles of the common wheat; and greasing their tips produced much less effect than in, the case of the oats. After 22 h., 5 of them had come into contact with the bottom of the sieve; 2 had moved towards it 10° and 15°, and one alone remained perpen- dicular. Not one of the very numerous ungreased radicles failed to come into close contact with the sicve. These trials were made on Nov. 28th, when the temperature was only 4°°8 C, at 10 am. We should hardly have thought this case werth notice, had it not been for the following circumstance. In the beginning of October, when the temperature was considerably higher, viz., 12° to 18° C., we found that only a few of the ungreased radicles became bent towards the sieve; and this indicates that sensitiveness to moisture in the air is increased by a low temperature, as we have seen with the radicles of Vicia faba relatively to objects attached to their tips. But in the present instance it is possible that a diffvrence in the dryness

of the air may have caused the difference, in the results at the two periods. Finally, the facts just given with respect to Phaseolus multijlorus, Vicia faba, and Avena sativa show, as it seems to us, that a layer of grease spread for a length of 1} to 2 mm. over the tip of the radicle, or the destruction of the tip by caustic, greatly lessens or quite annuls in the upper and exposed part the power of bending towards a neighbouring source of moisture. We should bear in mind that the part which bends most, lies at some little distance above the greased or oauterised tip; and that the rapid growth of this part, proves that it has not been injured by the tips having been thus treated. In those cases in which the radicles with greased tips became curved, it is possible that the layer of grease was not sufficiently thick wholly to ex- clude moisture, or that a sufficient length was not thus protected, or, in the case of the caustic, not destroyed. When radicles with greased tips are lett to grow for several days in damp air, the grease is drawn out into the finest reticulated threads and dots, with narrow portions of the surface left clean. Such portions would, it is probable, be able to absorb moisture, and thus we can account for several of the radicles with greased tips having become curved towards the sieve after an interval of one or two days. On the whole, we may infer that sensitiveness to a difference in the amount of moisture in the air on the two sides of a radicle resides in the tip, which transmits some influ- ence to the upper part, causing it to bend towards the source of moisture. Consequently, the movement is the reverse of that caused by objects attached to one side of the tip, or by a thin slice being cut off, or by being slightly cauterised. In a future chapter it will be shown that sensitiveness to the attraction of

gravity likewise resides in the tip; so that it is the tip which excites the adjoining parts of a horizontally extended radicle to bend towards the centre of the earth. Srconpary RADICLES BECOMING VERTICALLY GE&EO- TERMINAL Part of THE Primary RaDIcLe. Sachs has shown that the lateral or secondary radicles of the bean, and probably of other plants, are acted on by geotropism in so peculiar a manner, that they grow out horizontally or a little inclined down- wards ; and he has further shown* the interesting fact, that if the end of the primary radicle be cut off, one of the nearest secondary radicles changes its nature and grows perpendicularly downwards, thus replacing the primary radicle. We repeated this experiment, and planted beans with amputated radicles in friable peat, and saw the result described by Sachs; but generally two or three of the secondary radicles grew perpendicularly downwards. We also modified the experiment, by pinching young radicles a little way above” their tips, between the arms of a U-shaped piece of thick leaden wire. The part pinched was thus flattened, and was afterwards prevented from growing thicker. Five radicles had their ends cut off, and served as controls or standards. Eight were pinched ; of these 2 were pinched too severely and their ends died and dropped off; 2 were not pinched enough and were not sensibly affected ; the remaining 4 were pinched sufficiently to check the growth ot the terminal part, but did not appear otherwise injured. When the U-shaped wires were removed, after an

interval of 15 days, the part beneath the wire was found to be very thin and easily broken, whilst the part above was thickened. Now in these four cases, one or more of the secondary radicles, arising from the thickened part just above the wire, had grown perpendicularly downwards. In the best case the primary radicle (the part below the wire being 14 inch in length) was somewhat distorted, and was not half as long as three adjoining secondary radicles, which had grown vertically, or almost vertically, downwards. Some of these secondary radicles adhered together or had become confluent. We learn from these four cases that it is not necessary, in order that a secondary radicle should assume the nature of a primary one, that the latter should be actually amputated; it is sufficient that the flow of sap into it should be checked, and consequently should be directed into the adjoining secondary radicles; for this seems to be the most obvious result of the primary radicle being pinched between the arms of a U-shaped wire.

This change in the nature of secondary radicles is clearly analogous, as Sachs has remarked, to that ~vhich occurs with the shoots of trees, when the leading one is destroyed and is afterwards replaced by one or more of the lateral shoots ; for these now grow upright instead of sub-horizontally. But in this latter case the lateral shoots are rendered apogeotropic, whereas with radicles the lateral ones are rendered geotropic. We are naturally led to suspect that the same cause acts with shoots as with roots, namely, an increased flow of sap into the lateral ones. We made some trials with Abies communis and pectinata, by pinching with wire the leading and all the lateral shoots excepting one. But we believe that they were too old when experi- mented on; and some were pinched too severely, and

some not enough. Only one case succeeded, namely with the spruce-fir. The leading shoot was not killed, but its growth was checked; at its base there were three lateral shoots in a whorl, two of which were pinched, one being thus killed; the third was left untouched. These lateral shoots, when operated on (July 14th) stood at an angle of 8° above the horizon ; by Sept. 8th the unpinched one had risen 35°; by Oct. 4th it had risen 46°, and by Jan. 26th 48°, and it had now become a little curved inwards. Part vf this rise of 48° may be attributed to ordinary growth, for the pinched shoot rose 12° within the same period. It .thus follows that the unpinched shoot stood, on Jan. 26th, 56° above the horizon, or 34° from the vertical; and it was thus obviously almost ready to replace the slowly growing, pinched, lead- ing shoot. Nevertheless, we feel some doubt about this experiment, for we have since observed with spruce-firs growing rather unhealthily, that the lateral shoots near the summit sometimes become highly inclined, whilst the leading shoot remains apparently sound.

A widely different agency not rarely causes shoots which naturally would have grown out horizontally to grow up vertically. The lateral branches of the Silver Fir (A. pectinata) are often affected by a fungus, Micidium elatinum, which causes the branch to enlarge into an oval knob formed of hard wood, in one of which we counted 24 rings of growth. According to De Bary,* when the mycelium penetrates a bud be- gianing to elongate, the shoot developed from it grows vertically upwards. Such upright shoots after-

* Sce his valuable article in are culled in German “ Hexen ‘Bot. Zeitung,’ 1867, p. 257, on — besen,” or “ witeh-brooms,” these monstrous growths, which ards produce lateral and horizontal branches; and they then present a curious appearance, as if a young fir-tree had grown out of a ball of clay surrounding the branch. These upright shoots have manifestly changed their nature and become apogeotropic; for if they had not been affected by the Atcidium, they would have grown out horizontally like all the other twigs on the same branches, This change can hardly be due to an increased flow of sap into the part; but the presence of the mycelium will have greatly dis- turbed its natural constitution.

According to Mr. Meehan,* the stems of three species of Euphorbia and of Portulaca oleracea are “normally prostrate or procumbent;” but when they are attacked by an AXcidium, they “assume an erect habit.” Dr. Stahl informs us that he knows of several analogous cases; and these seem to be closely related to that of the Abies. The rhizomes of Sparganium ramosum grow out horizontally in the soil to a con- siderable length, or are diageotropic; but F. Elfving found that when they were cultivated in water their tips turned upwards, and they became apogeo- tropic. The same result followed when the stem of the plant was bent until it cracked or was merely much bowed.t

No explanation has hitherto been attempted of such cases as the foregoing,—namely, of secondary radicls growing vertically downwards, and of lateral shoots growing vertically upwards, after the amputation of + See F. Elfving’s interesting paper in ‘Arbeiten Bot. Institut., in Wirzburg,’ vol. ii. 1880, p. 489. Carl Kraus (Triesdorf) had pre- viously observed (‘ Flora,’ 1878, p. #24) that the underground shoots of Triticum repens bend vertically up when the parts above ground are removed, and when the rhizomes are kept partly im merscd in water.

the primary radicle or of the leading shoot. The following considerations give us, as we believe, the clue. Firstly, any cause which disturbs the con- stitution * is apt to induce reversion; such as the crossing of two distinct races, or a change of con- ditions, as when domestic animals become feral. But the case which most concerns us, is the frequent appearance of peloric flowers on the summit of a stem, or in the centre of the inflorescence,—parts which, it is believed, receive the most sap; for when an irregular flower becomes perfectly regular or peloric, this may be attributed, at least partly, to reversion to a primi- tive and normal type. Even the position of a seed at the end of the capsule sometimes gives to the seedling developed from it a tendency to revert. Secondly, reversions often occur by means of buds, independently of reproduction by seed; so that a bud may revert to the character of a former state many bud-generations ago. In the case of animals, reversions may occur in the individual with advancing age. Thirdly and lastly, radicles when they first protrude from the seed are always geotropic, and plumules or shoots almost always apogeotropic. If then any cause, such as an increased flow of sap or the presence of mycelium, disturbs the constitution of a lateral shoot or of a secondary radicle, it is apt to revert to its primordial state; and it becomes either apogeotropic or geotropic, as the case may be, and consequently grows either vertically upwards or downwards. It is indeed pos-

* The facts on which the fol- xiv. On pvloric flowers, chap. lowing conclusions are founded — xiti, p.#2; and see p. 337 on thei are given in ‘The Variation of — position on the plant. With Animals and Plants under Domes- respect to seeds, p. 340. On re- tieation,’ 2nd edit 1875. Ou the version by means of buds p. 138 rauses leading to reversin sce — chup. xi vol. i. , chap, xii. vol. ii, and p. 59 chap. sible, or even probable, that this tendency to reversion may have been increased, as it is manifestly of service to the plant.

A part or organ may be called sensitive, when its irritation excites movement in an adjoining part. Now it has been shown in this chapter, that the tip of the tadicle of the bean is in this sense sensitive to the contact of any small object attached to one side by shellac or gum-water; also to a slight touch with dry caustic, and to a thin slice cut off one side. The radicles of the pea were tried with attached objects and caustic, both of which acted. With Phaseolus multiflorus the tip was hardly sensitive to small squares -of attached card, but was sensitive to caustic and to slicing. The radicles of Tropeclum were highly sen- sitive to contact; and so, as far as we could judge, were those of Gossypium herbucewm, aud they were certainly sensitive to caustic. ‘The tips of the radicles of Cucurbita ovifera were likewise highly sensitive to caustic, though only moderately so to contact. Ra- phanus sativus offered a somewhat doubtful case. With Atsculus the tips were quite indifferent to bodies attached to them, though sensitive to caustic. Those of Quercus robur and Zea mays weré highly sen- sitive to contact, as were the radicles of the latter to caustic. In several of these cases the difference in sensitiveness of the tip to contact and to caustic was, as we believe, merely apparent ; for with Gossypium, Raphanus, and Cucurbita, the tip was so fine and flexible that it was very difficult to attach any object to one of its sides. With the radicles of A‘sculus, the tips were not at all sensitive to small bodies attached to them; but it does not follow from this

fact that they would not have been sensitive to scme- what greater continued pressure, if this could have been applied. The peculiar form of sensitiveness which we are here considering, is confined to the tip of the radicle for a length of from 1mm. to 1-5 mm. When this part is irritated by contact with any object, by caustic, or by a thin slice being cut off, the upper adjoining part of the radicle, for a length of from 6 or 7 to even 12 mm., is excited to bend away from the side which has been irritated. Some influence must there- fore be transmitted from the tip along the radicle for this length. The curvature thus caused is generally symmetrical. The part which bends most apparently coincides with that of the most rapid growth. The tip and the basal part grow very slowly and they bend very little.

Considering the widely separated position in tbe vegetable series of the several above-named genera, we may conclude that the tips of the radicles of all, or almost all, plants are similarly sensitive, and transmit an influence causing the upper part to bend. With respect to the tips of the secondary radicles, those of Vicia faba, Pisum sativum, and Zea mays were alone observed, and they were found similarly sensitive. In order that these movements should be properly displayed, it appears necessary that the radicles should grow at their normal rate. If subjected to a high temperature and made to grow rapidly, the tips seem either to lose their sensitiveness, or the upper part to lose the power of bending. So it appears to be if they grow very slowly from not being vigorous, or from being kept at too low a temperature, also when they are forced to germinate in the middle of the winter.

The curvature of the radicle sometimes occurs within from 6 to 8 hours after the tip has been irritated, and almost always within 24 h., excepting in the case of the massive radicles of Adsculus. The curva- ture often amounts to a rectangle,—that is, the ter- minal part bends upwards until the tip, which is but little curved, projects almost horizontally. QOcca- sionally the tip, from the continued uritation of the attached object, continues to bend up until it forms a hook with the point directed towards the zenith, or a loop, or even a spire. After a time the radicle apparently becomes accustomed to the irritation, as occurs in the case of tendrils, for it again grows down- wards, although the bit of card or other object may remain attached to the tip.