Plant Autographs and Their Revelations

strictly coincide with the daily change of light and dark¬ ness. The movement of the Water-Lily petals is, therefore, not essentially dependent on variations of light. It was shown that the movement of its petals was not affected by gravity nor by light. On what then does the movement depend? To ascertain this we have to take a continuous record of the movement of its petals. They are closed in the day-time; we want to know the exact moment when they begin their ‘waking’ movement, the time when this movement is most rapid and when the flower becomes fully expanded.

After this expansion, there must be a time for the reverse movement or closure of the petals. When does this begin, at what rate does it proceed, and when does the flower completely close its petals in ‘sleep’ ? The only other fluctuating element which may possibly affect the movement of the petals is the daily variation of temperature. In summer the minimum temperature is attained about 6 a.m. After this, the rise is very rapid, the maximum being attained at about 2.30 p.m. The temperature then begins to fall until the minimum is reached next morning. The time taken for a rise from the mini¬ mum to the maximum is 8^ hours ; the period of fall through the same range is, on the other hand, 15/4 hours. The rate of rise is therefore far more rapid than the rate of fall. In winter the minimum temperature is attained half an hour later and the maximum half an hour earlier. In order to determine the influence of temperature on the movements of the flower, it is necessary to obtain a daily record, both of the petals and of the change of tempera¬ ture throughout the twenty-four hours.

By means of the Automatic Recorder the two records are taken simultaneously on the same plate. In the com¬ plete record (fig. 65) the upper curve shows the daily varia¬ tion of temperature, and the lower curve the movement of the petal. The corresponding thick dots in the upper and the lower curves indicate the precise hour at which the two essential changes take place. It will now be seen how astonishingly parallel is the curve of the movement of the flower to the curve which represents the variation of temperature. There can, there¬ fore, be no doubt that the cause of the opening and closing of the flower is the diurnal change of temperature. The flower is in a position of sleep during the day; a rapid fall

of temperature occurs from 6 p.m. and the petals begin to open, at first slowly, then vel*y rapidly. The flower is com¬ pletely open and fully expanded by 10 p.m. Though the temperature continues to fall, there is no further possibility of expansion beyond the maximum. At about 6 a.m. the temperature begins to rise, so the reverse movement of closure sets in. The flower continues to close very rapidly till the closure of ‘sleep’ becomes complete by about

about by a rise of temperature, the opening by a fall. The explanation of the movement is that a young flower is still in a state of growth, and rise of temperature accelerates while a fall of temperature retards it. Now in the petal of the Lily, the two sides differ in sensibility, just as we found the upper and lower sides of the pulvinus of Mimosa to be unequally sensitive. In the Indian Water-Lily it is the outer side which is the more sensitive. Hence, during rise of temperature, the outside grows faster than the in¬ side, thus producing a closing movement. During the fall of temperature, the reverse movement takes place, since the retardation of growth is greater in the more sensitive outer side.

In the European Lilies it is the inner side that is rela¬ tively the more sensitive. These flowers are compelled to reverse the order of the reactions of the Indian Water- Lily, by opening in the day-time and closing at night. They cannot, therefore, be accused of moon-worship. They have the healthy habit of normal humanity to sleep at night and keep awake in the day-time. Others turn night into day, and make up for their long night-watch by sleeping it off' in the day-time!

As an example of an organ specially sensitive to light we may take the leaflet of the Indian plant Cassia alata. These leaflets remain tighly closed during the night, but from early morning onwards they begin to open and remain widely spread out throughout the day (fig. 66). So sensitive are the leaflets that cloud-shadows cause them to begin to close. In order to show that the daily opening and closing movements of this plant are entirely due to alternation of light and darkness, a continuous record was taken from 4 p.M. till noon the next day. The first thick dot was marked at 4 p.m. : the succcessive thick dots are at intervals of an hour, the smaller dots being fifteen minutes apart. It will be noticed that the movement of closure of the leaf-

lets started at 5 p.m. when the light began to wane. The leaflets became completely closed by 9 p.m. and remained closed till 5 A.M. next morning. After this, they began to open again and became completely outspread by 9 a.m., and remained so till late in the afternoon (fig. 67). Then the same cycle repeated itself. The fact that these move¬ ments are entirely due to the action of changes in light and not to variation of temperature can easily be proved by placing a black cloth over the plant during the middle of the day. The temperature remains the same; yet because of the artificial darkening, the leaflets rapidly close.

The plant absorbs from the soil water holding food-mate¬ rial in solution. The distribution of the absorbed liquid, that is the rise of the sap, enables the plant to maintain the cells in that state of turgor without which the growth of the plant and its various life-movements would be arrested. But how is the sap raised to the top of a tree? This question has perplexed scientific investigators for more than two hundred years ; it has been argued, on the one hand, that it is wholly due to the action of physical forces, and on the other, that it is brought about by some activity of the living tissues. For reasons to be presently explained, the actual consensus of opinion is in favour of the physical theory, but the evidence to be here adduced proves that view to be quite untenable.

Let us take the instance of a tall tree with numerous leaves. What are the physical forces which may be con¬ cerned in raising the sap from the roots to the leaves? Transpiration, that is the exhalation of water-vapour, is constantly taking place from these leaves and a partial vacuum is produced in the wood-vessels which run through the plant from top to bottom. The atmospheric pressure will consequently force the water up the plant; but the greatest height to which water can be raised in this manner is only 34 feet, the height of the water-barometer. The Palms, however, grow to a height often exceeding 100 feet; but even the Palm is a pigmy compared to some of the giants like Eucalyptus amygdalina with a height of 450 feet. The theory of atmospheric pressure is, therefore, inadequate, as is also that of capillarity.

The theory of osmotic action has also been invoked in explanation of the phenomenon. When a semi-permeable bag containing a strong sugar-solution is dipped in v^ater, the water passes into the bag containing the strong solu¬ tion. The vegetable cells are like so many bags containing strong solutions, and they will, therefore, take up water accessible to them. The cells in the root thus take up water from the soil, the next cell higher from the one below it, and so on in progressive series upwards.

But this osmotic process is exceedingly slow. Let us see what this means when the leaves at the top of a Eucalyptus are on the point of death from severe drought. We can well imagine the impatience with which the parched leaves would be waiting for the sap to reach them after rain had irrigated the roots. If the water-movement depended solely on osmotic action, then their prospect would be none too cheerful, for the shortest time in which water would reach the top by osmosis would be considerably over a year ! Among the upholders of the physical theory is Strasburger, an eminent plant-physiologist; but even he has been con¬ strained to say that ‘Osmotic forces act too slowly to be of any value, and, moreover, there is no fixed distribution of osmotic substances that would account for such a cur¬ rent.’ ^

There must, therefore, be some other agency at work to effect a more rapid propulsion of the sap. Another theory is that the rise is due to the action of a pull from above and a push from below exerted by the terminal organs, the leaf and the root. This theory is only partly physical, for both the pull and the push depend upon the vital activity of leaf and root respectively. The removal of water from leaves by transpiration is supposed to exert a pull along cohering columns of water in the vessels present in the wood. But the water-columns in the vessels are not continuous but are interrupted by air-bubbles ;

^ Strasburger, Text-Book of Botany (English Translation, p. 187). it is obviously impossible to employ ropes of water for haulage, particularly if they are severed here and there! So much for the pull from above. The push from below is supposed to be exerted by root-pressure. But in the Palm there is no detectable root-pressure, yet the sap rises to a height of more than a hundred feet. Again, during active transpiration, when the need of the tree is greatest, the root-pressure, instead of being positive, is actually nega¬ tive. The fact that neither the leaf nor the root is absolutely necessary for the propulsion of sap will be demonstrated later, when I show that the sap-movement persists even after complete removal of the root and the leaves.

The various physical theories having proved unsatisfac¬ tory, there remains the question whether the activity of liv¬ ing cells might not be instrumental in effecting the propul¬ sion of sap. This physiological theory, however, received a severe blow from some inconclusive experiments of Stras- burger, which led him to the conclusion that the ascent of sap is not affected by the application of poison which must necessarily kill all living tissues. My results, to be pres¬ ently described, lead to a conclusion diametrically opposite to Strasburger’s.

The following experiments on the effect of stimulants and poisons are quite decisive in proving that the propul¬ sion of sap is essentially due to the activity of living cells. A cut stem of Chrysanthemum coronarinm is subjected to drought; the plant doubles over, the leaVes shrink and crumple up; in fact, the plant seems dead. But irrigation with water containing traces of a stimulant brings about a marvellous transformation; there is now an active rise of sap by which the original turgor is restored. The bent stem straightens up and the leaves spread out in their original vigour, as seen in the photograph reproduced (fig. 68); complete recovery was, in this case, found to

take place in as short a time as fifteen minutes. In sharp contrast with this was the parallel experiment, in which the drooping stem was irrigated with poisonous formalde¬ hyde solution. The specimen never recovered, but under- Fig. 68. Full erection of a cut shoot with drooping leaves on application of water at the cut end. {Chrysanthemum.) went a complete collapse into a huddled mass of dying tissue (fig. 69). I introduced certain modifications in the next experi¬ ment; the specimen employed was also different. An erect stem of Centmirea was placed in a vessel P containing a poisonous solution of potassium cyanide, while a drooping

Fig. 69. Photographs of drooping cut shoot of Chrysanthemum placed in solution of Formaldehyde, which caused increased drooping, instead of full erection due to ascent of water as in fig. 68. Fig. 70. Erect stem placed in poisonous and drooping stem in stimulat¬ ing solution (left). The poisoned stem droops and dies, while the drooping stem becomes stem of the same plant was placed in the vessel S, which contained a stimulant. The opposite effects of poisonous and stimulating solutions are strikingly shown in the pair of photographs to the right. The erect specimen under poison shows a complete collapse, while the drooping stem under stimulant exhibits a vigorous recovery (fig. 70).

The evidence adduced suffices to prove that the propul¬ sion of sap is essentially due to the activity of living tissues. But a vague assumption of living activity is not a complete explanation of the phenomenon. It is necessary further to determine the character of the underlying activity, how that activity is initiated, and by what means a definitely directioned transport of sap is maintained. In order to obtain a complete solution of this problem, I had to devise diverse methods of experimentation and various instruments. The results obtained led to the con¬ clusion that the sap in the plant is propelled by a mechan¬ ism which is essentially similar to that which maintains the circulation of blood in the animal.

The flow of blood in a higher animal is maintained by the automatic and ceaseless pulsations of a contractile organ which we call its heart. We must, however, adopt a generalised idea of the heart and its mechanism, for it would not be reasonable to seek in the plant for an organ so highly complex and centralised as, say, the human heart. The correspondence with the plant is rather to be sought in the lower animals, in which, as in Amphioxus, the heart is an elongated tubular organ, the contained nutrient fluid being propelled by waves of peristaltic contraction which sweep forward. In the embryo of even the higher animals, the heart is an elongated tube. The essential characteristic of the cardiac tissue of the animal is its rhythmic pulsation, the rapidity of which is appropriately modified under defi¬ nite conditions.

Thus under certain stimulating drugs the heart beats faster, and pumps the blood at a quicker rate, ensuring a more rapid flow; a depressant induces a precisely opposite reaction. A certain amount of internal tension is necessary to initiate the pulsation of the heart; thus the quiescent heart of a snail is made to pulsate by subjecting it to an increased intracardiac pressure. The activity of the heart is increased within limits under a rise of temperature, and decreased by application of cold.

A small dose of a narcotic like ether accelerates the car¬ diac activity. A strong narcotic like chloroform causes an immediate stimulation followed by depression and arrest under its continued action. I will now show that any condition which enhances the activity of the heart and thus increases the rate of flow of blood, also increases the rate of flow of sap; and con¬ versely, that depressing agents lower or arrest the flow in both. The difficulty to be overcome was the fact that no satisfactory method had hitherto been available for the de¬ tection and measurement of the normal rate of ascent of sap and its induced variations.

I succeeded in removing the difficulty by utilising the leaf as an indicator of the movement of sap. In a potted plant, drought is found to produce, sooner or later, a droop¬ ing of the leaves, whereas after irrigation they become erect. Moreover, any agent which increases the activity of pumping the sap, also causes a rapid erection of the leaf ; diminution of the pumping activity by a depressant pro¬ duces, on the other hand, a fall of the leaf. These move¬ ments are, however, too slight to be easily observed ; they become conspicuous when highly magnified, as by the fol¬ lowing device.

This is a very sensitive apparatus for the detection of changing rates in the ascent of sap. The leaf is attached by a thin silk thread to the recording lever, which magnifies the movement of the leaf from ten to a hundred times. The record is taken on a smoked glass plate. The lever, by means of an electro-magnetic device, is made to tap successive dots on the moving recording plate at intervals with the leaf of a cut shoot is practically the same as that of an intact plant with roots; a cut shoot obviously offers greater facilities for manipulation.

mounted on a revolving rod, which can be raised or lowered by means of a handle. The cut end of the shoot may thus be acted upon in rapid succession by a chemical depressant or a stimulant, or by cold or warm water. To demonstrate the similarity of action of the propulsive mechanism in plant and animal, I will give a detailed description of the opposite effects produced by depressing Fig. 72. Alternate arrest and enhancement of ascent of sap by chemical depressant and stimulant.

and stimulating agents. The leaf was in a balanced hori¬ zontal position. Application of potassium bromide solution produced so great a depression that the leaf exhibited a rapid fall. The effect of the application of a small dose of camphor, which is a stimulant, was the arrest of the fall and the subsequent rise of the leaf (fig. 72). Hence it may be inferred that the supply of sap to the leaf was diminished in the one case and increased in the other, and that the reagents must have affected living cells by which the supply is maintained.

The effect of drought was exhibited by the fall of the leaf. Here again the propulsion of sap must have been interfered with, probably by a diminution of the internal hydrostatic pressure in the propulsive cells. The effect of alternate application of cold and warm water in successive depression and enhancement of the Fig. 73. Effect of alternate application of cold (C) in depression (down- curve) and of warmth (H) in enhancement (up-curve) of rate of ascent.

rate of ascent of sap was clearly demonstrated in the record (fig- 73)- Dilute ether was found to produce an enhancement in the rate of ascent and a rapid erection of the leaf. Stimu¬ lation followed by depression under the action of chloro¬ form was exhibited by the preliminary erection of the leaf followed by a rapid fall. Some of the important facts established in this chapter may now be recapitulated. The arrest and permanent abolition of the ascent of sap in a plant treated with poison

prove it to be due to the activity of living tissue. The alternate arrest and renewal of the ascent under cold and warmth, and the characteristic enhancement or depression of the rate under cardiac depressants and stimulants, further show that the mechanism for the propulsion of fluids is fundamentally similar in plant and animal. Physical mechanism could not have in any way manifested these characteristic reactions. The experiments described in the last chapter prove that there is an active tissue somewhere in the interior of the plant, the pulsation of which effects the propulsion of the sap, just as the pulsation of the heart maintains the circula-

Fig. 74. The Electric Probe for the localisation of the Pulsating Layer. The point of the Probe enters the stem at A, the second electric contact being made with a distant leaf. The figure to the right is an enlarged view with the micrometric screw for the gradual introduction of the Probe into the tissues of the plant. tion of the blood in the animal. There must, therefore, be something like a primitive ‘heart’ in the plant, but not so centralised and highly differentiated as in higher animals. In the lower types of animal, as also in the embryo of the higher, the heart is an elongated organ, the contained nutrient fluid being propelled forward by peristaltic con¬ traction. The propulsion of sap in plants, I find, is due to