Comparative Electro-Physiology: A Physico-Physiological Study

203 gives a series of such responses. It was said at the beginning that the responses of digestive organs were likely to be multiple. This is seen to be true even under the moderate stimulus applied in the present case. But under the action of stronger stimulus, such as that of a thermal shock, the response is found to consist of a long and multiple series, records of which will be seen later. Another peculiarity to be noticed, in the series of re- sponses given in fig. 203, is that the base-line of the record

trends upwards. This indicates that the glandular surface, by the residual effect of stimulation, is being rendered more and more galvanometrically negative. This explains why the internal surface of the stomach has been found by different observers to be negative, a condition of more or less persistent negativity being thus clearly due to the ex- citatory after-effect of preparation. Had it not been for the exceptional opportunity afforded by the open pitcher of Nepeuthe, it would have been impossible to make galvano- metric connections with the intact inner glandular surface and thus to ascertain that such a surface is naturally gal- vanometrically positive.

I may here point out the very interesting medics tion of response which occurs in the same specimen under a long-continued series of stimulations. This modification, due to fatigue so-called, makes its appearance first in dimi- nution of the height of the responses. Some of the con- stituent multiple responses due to a single stimulus are then found to be reversed to positive, and after this they show a tendency to become more or less completely reversed.

It is also interesting to find that the same modifications make their appearance, in the same order, in those pitchers which have been subjected to continuous stimulation, to a greater or less extent, by the supply of insects. That is to say, a pitcher containing a few insects is found to give responses, the multiple constituents of which are sometimes positive and sometimes negative. This intermediate phase is seen well illustrated in the record given in fig. 204. But in the pitcher whose inner surface is already thickly coated with insects, and which has long been exposed to the continuous action of such stimulation, the characteristic response is found to be the reverse of that of the fresh specimen. — It will be seen from the record in fig. 205 that in such a case the individual effect of a single stimulus is a series of mul- tiple responses which are positive. In this record a curious

effect is again seen, that of the shifting om the base-line, now ae downwards. This indicates an increasing positivity of the glandular surface. The results which have thus been described, in the case of the fresh pitcher, and of one subjected for a long period to the stimulus of food, are fully compatible, it will be observed, with the theory of digestion as a diphasic process, in which galvanometric negativity is associated with a predominant secretion, and the subsequent galvanometric positivity with a. predominant absorption by the glandular membrane.

Fic. 205. Photographic Record of Re- sponses of Pitcher in Third Stage, the whole Glandular Surface thickly Fic.. 204. Photographic Record Coated with Insects. Stimuli applied of Responses of Pitcher in at Intervals of two Minutes Intermediate Stage, having Attracted a Few Insects The response here is in the positive phase, direction of current being from non- Note here the occurrence of two glandular to glandular. Notealso the phases in constituent responses, multiple character of responses to positive being predominant. single stimuli.

It is also important to notice that while in the fresh condition the glandular surface is positive, and in the moderately stimulated condition negative, yet positivity of the glandular surface is not always to be taken as a sign of its fresh condition. For we have here seen that under long- continued stimulation, the electrical condition is apt to be reversed to one of positivity, It has been stated that on account of the highly excitable nature of the digestive organ, a single stimulus, if strong,

Fic. 206. Multiple Response of Pitcher of Mefenthe, in First or Fresh Stage, to Single Strong Thermal Shock The constituent responses are both negative and positive, the former being stronger. would give rise in it to a multiple series of responses. The two following records (figs. 206 and 207) illustrate this fact in two different speci- mens which were in somewhat different con- ditions. The stimulus employed in each case was a _ single strong thermal shock, and the multiple responses were found to persist, in both, for quite an hour. In the first of these figures, the constituent responses of the series were both negative and positive, ye, 207, Multiple Response of Pitcher of the former being pre- Nepenthe, in Third Stage, to Single Strong dominant. In the second gna ae

The constituent responses are here - pre- record (fig. 207 ) the dominantly positive. positive phase is pre- dominant in the responses, and the trend of the base-line down- wards shows increasing positivity of the glandular surface. In taking up this investigation on the pitcher of Vepenthe it appeared to me that much light would be thrown, by the study of this simple organ, on the many: difficulties connected with the response of the more complex digestive organs of the animal. This surmise has proved to be fully justified, for in the experiments which I have carried out in the latter field, the results are a mere repetition of these typical effects seen in Wepenthe under corresponding circumstances.

Before passing from Nepenthe to the study of digestive tissues in animals, it will be well to deal here with the more complex type of vegetal digestive organ seen in the plant Drosera. 1 took for my experiment a specimen of the Indian Dvrosera longifolia, the upper surfaces of whose leaves are covered, as is well known, with glandular Fic. 208. Photographic Record of Responses : in Fresh Leaf of Drosera to Equi-alternating tentacles. Here, as ‘5 Electrical Shocks the case of the pitcher The first series show normal responses. Current J h from upper glandular to lower non-glandular of Nepentne, the re-

surface. In the second series normal response sponse of leaves which is reversed to positive, after tetanisation, T. are fresh and have not been subjected to previous excitation, is by induced negativity of the glandular surface, and this is reversed to positive under long-continued stimulation. These two phases are seen in figure 208, in which the first series is a record of normal responses of galvanometric negativity, to equi-alternating shocks applied at intervals of one minute; and the second, the reversed responses exhibited by the same leaf, to the same stimulus, when it has, in the meantime, been subjected to tetanising shocks for three minutes continuously. It is

curious and interesting to note here, as in the case of Nepenthe, the trend of the base-line up, when the response is the normal negative, and down when it is the reversed positive, indicating in the one case increasing negativity, and in the other increasing positivity. As in the Mepenthe, so also in the leaf of Drosera, specimens which are not fresh—that is to say, previously unexcited——are apt to exhibit the positive phase of response. I give below a series of multiple responses (fig. 209) induced in such a leaf by a strong stimulation. The stimulus was in this case given by sectioning the leaf, and the response therefore illustrates the fact that preparation itself acts as a stimulus. In the present case, electrical con- nections with the galvanometer, were made with the upper and lower surfaces of the leaf on the plant, intact. On now cutting the petiole across, a long series of multiple responses, lasting for about 45 minutes, was found to be set up. These pulsations were at first rapid, and then slowed down gradually, the average period of a r pae, Pestommpnic Bacol single pulsation being about 30 of Drosera in Positive Phase seconds. Only a portion of the ‘Stimulus was caused here by

Having thus seen the typical responses exhibited by the digestive organs of plants, we shall now pass to the consideration of the reactions induced in animal stomachs. Here, again, two different subjects of inquiry arise, the direction, namely, of the natural current of rest, and that of the action or responsive current. As regards the first of these, it will be remembered that Rosenthal found it to be strongly ‘ingoing’—that is to say, from the mucous to the

muscular coats of the stomach. From this it was supposed, as we have seen, that the mucous coat of the stomach of the frog had the same electro-motive reaction as its outer skin. We shall find, however, that there is in reality no such similarity between the two, inasmuch as, while the excitatory reaction makes the outer skin galvanometrically positive, its effect on the mucous surface under normal conditions is to induce galvanometric negativity. In the case of Nepenthe, further, we have seen that the natural current of rest is from the non-glandular outer to the glandular inner surface, and that this is liable to reversal, as an excitatory after-effect of preparation. The i ingoing current, therefore, observed in the preparation of frog’s stomach, is to be regarded, not as the natural current of rest, but as the excitatory after-effect due to isolation.

With regard, next, to the current of action, Biedermann states that direct electrical excitation, by rapidly alternating shocks, induces a negative variation usually preceded by a positive swing. Since the so-called current of rest is ingoing, a ‘negative variation’ of it evidently means an outgoing current—that is to say, galvanometric positivity of the mucous coat. Hence the responsive action of the mucous coat, as described by Biedermann, is a transient negativity followed by positivity.

In dealing with this question of the electrical response of the digestive organ, we must be prepared, as the result of previous experiments on plants, to meet with variations of the excitatory effect, due to the phasic condition of the tissue. And first, for the clear demonstration of the effect of ex- citation on the mucous surface, uncomplicated by changes induced at the second contact, I employed the Rotary Method of Mechanical Stimulation of the given area. The rotating electrodes were applied to the inside of a properly mounted frog’s stomach, and experiment commenced some time after the cessation of the multiple response due to preparation. The following record (fig. 210) exhibits the first four of these responses to individual mechanical stimuli,

applied at intervals of one minute. The responsive variation took place by the induced galvanometric negativity of the excited area. Under long-continued stimulation fatigue was found to be induced, the responses becoming diminished and even tending to a reversal from the normal to positive. In order to show that the inner mucous surface is relatively more excitable than the muscular coat, I next subjected the two to simultaneous excitation by equi-alter- nating electrical shocks, And for the sake of establishing a

Fic, 210, Photographic Fic. 211, Photographic Record Record of Normal of Normal Negative Re- Negative Responses sponses of Stomach of Tor- of Frog’s Stomach to toise to Stimulus of Equi- Mechanical Stimula- alternating Electric Shocks tion applied at Intervals of One generalisation as to the reaction of the stomach, I now took a different specimen—namely, the stomach of tortoise. The responses in the figure (fig. 211) showed relative galvano- metric negativity of the inside of the stomach.

We have seen that fresh vegetable stomach responds by normal negativity, but that, under continuous stimulation, a phasic change is induced, by which response is reversed to positive (cf. fig. 208). I shall next demonstrate the corre- sponding effect in the animal stomach. Taking.a preparation of the stomach of gecko, I obtained normal responses, whose direction was from the glandular internal to the muscular external surface. After an intervening period of tetanisation, however, the responses are seen to be reversed (fig. 212).

The next record has been selected for the purpose of showing the gradual process of transition from the normal negative to the reversed positive response. The specimen taken was frog’s stomach. At the commencement of the experiment the galvanometer spot was quiescent, but when the specimen was subjected to a single strong thermal shock, a prolonged series of multiple responses was initiated, per- sisting for more than an hour, Of this series I here re- produce four different portions (fig. 213). The first of these (a) con- sists of pulses of gal- vanometric negativity of the internal surface. The recoveries are here incomplete, and _ the base-line shifts upwards, showing an increasing negativity of that sur- face. The negative pulses are then reversed Fic. 212, Photographic Record of Normal to positive, through an

Response in Stomach of Gecko to Equi- intermediate di-phasic of this pronouncedly positive response (c) the base line is horizontal. It then begins to shift downwards (@), thus exhibiting a decreasing negativity—or increasing positivity—of the internal surface. In this periodic variation of the electrical condition we have a significant parallel to the records which we have already seen in Wepenthe and in Drosera (figs. 203, 205, 208, and 209). It has already been pointed out that, in view of the functional peculiarities of the digestive organ, it might be ex- pected that the alternate reactions of secretion and absorption would neither of them be single and spasmodic, but each long-

sustained. In this connection it is suggestive that the digestive organs should show so strongly marked a characteristic of multiple response. It would thus appear, as already said, that mechanical stimulation during ingestion of food gives rise to the responsive reaction of secretion, evidenced electrically by response of galvanometric negativity of the internal surface. There then sets in the opposite phase, associated with the Fic. 213. Photographic Record of Multiple Responses in Stomach of Frog to a Single Strong Thermal Shock

Four parts are given of this long series. (a) Negative series; (4) Alter- nating negative and positive constituent responses ; (c) Positive series ; (Z) Positive series. Note trend of base-line upwards in negative a, and downwards in positive d. reversal of electrical response, probably indicating the ab- sorptive process. This reversal of response, to galvanometric positivity, may be the work of three different factors, which may or may not be mutually dependent. In the first place, we have seen that long-continued stimulation was apt of itself, other things being equal, to give rise to a reversal of response. Secondly, after secretion has reached its maxi- mum, the empty mucous cells in contact with fluid would naturally tend to reabsorb. And, lastly, we have seen that an increase of internal energy, in whatever way produced,

tends to give rise to a responsive reaction, whose sign is opposite to that of excitation—expansion instead of con- traction. Now such an increase of internal energy could not fail to be the result of the absorption of the chemically- dissolved food. Another interesting consideration to be remembered in connection with digestive organs is that periodically-acting forces give rise to an induced periodicity, which persists for | a time, even in the absence of the periodically-exciting cause. A well-known illustration of this is met with in the nycti- tropic movements, so-called, of plants, induced as these are by the periodic variation of night and day. These move- ments persist for a certain length of time, even when the plant is kept in continuous darkness. Similarly, animals accustomed to the supply of food at regular intervals would undoubtedly exhibit alternating phasic changes apparently

consequence of the original periodicity of the exciting cause. Such an organ, therefore, must necessarily exhibit periodic electrical variations. Parallelism between responsive reactions of root and digestive organ— Alternating phases of secretion and absorption—Association of absorptive process with ascent of sap—Electrical response of young and old roots—Different phasic reactions, as in pitcher of Mepenthe—Response to chemical stimulation— Different theories of ascent of sap—Physical versus excitatory theories— Objections to excitatory theory—Assumption that wood dead unjustified— Demonstration of excitatory electrical response of sap-wood—Strasburger’s

experiments on effect of poisons on ascent of sap—Current inference unjus- tified. : WE have seen in the last chapter that in the digestive pro- cess as a whole there must be alternating phases of secretion and absorption. The secretion of dissolving fluids, by which insoluble substances are rendered soluble, we found to take place under stimulation, and to be succeeded by a process of absorption, by means of which the now dissolved food- material found access into the organism. These functions, though seen characteristically in the digestive organs of animals, are also to be observed in some plants, such as the pitcher of Nepenthe, or the leaf of Drosera. Here, situated externally, we find what are practically open stomachs, digesting, as do those of animals, solid organic food. But plants in general have to depend on the supply of inorganic food-material, often presented in solid or insoluble forms, for their nourishment. In this case also it is obvious that the same sequence of solution by dissolving fluids, and subse- quent absorption, must be gone through. And the organ by which this takes place must evidently be the root. In this regard the well-known experiments on the corrosion of marble by the root of a growing plant are sufficient to show

that these organs secrete acids, by means of which insoluble substances are made soluble. It is equally clear, further, that the inorganic solids so dissolved are afterwards absorbed by the plant. Thus it will be seen that these alternating pro- cesses of secretion and absorption of food-material, as they take place in the vegetable organism, are not very different in their essential features from the ordinary phenomenon of digestion as known to us. The chief distinction between the two would now seem to lie in the fact that in the animal the supply of food is in the main organic, and in the plant inor- ganic. Even here, however, we meet with connecting links in the form of insectivorous plants, in whose case the organic supply is obtained by means of the digesting leaf, and the inorganic through the roots. We may regard digestion, therefore, in its widest sense, as a process of absorption of insoluble food rendered soluble, whether such food be organic or inorganic. Apparently, then, in the case of the plant the root functions as a digestive organ. But whether or not this analogy is merely superficial can only be determined by an experimental inquiry into the parallelism which may or may not exist between the various excitatory reactions of the root on the one hand and a typical digestive organ on the other.

In order to obtain the large quantity of inorganic material which is necessary to the nutrition of a tree, for instance, it is clear that fresh quantities of charged fluid must be con- stantly taken up. In order further that this process may be maintained continuously it must be possible to get rid of the useless water, which is accordingly passed off, chiefly from the transpiring leaves, in the form of vapour. The absorption of food and the ascent of.sap, or transpiration-current, would appear therefore to be related phenomena. I shall, in the course of the present and following chapters, then, take up in detail the consideration of these two aspects of the problem, which will thus constitute two main lines of inquiry:

(1) Whether or not the excitatory reaction of the root has any similarity to that of digestive organs in general; and (2) whether or not the ascent of sap is fundamentally due to similar excitatory reactions. With regard to the latter of these questions it may be stated here that the nature of the efficient cause of the ascent of sap is universally regarded, in plant physiology, as constituting a problem of the greatest obscurity. The various non-physiological theories which have hitherto been advanced are admitted to be inadequate, as we shall see later. We are thus confronted either with an insoluble problem or with the necessity of finding physiological reactions which will account for the ascent of sap.

As regards the latter of these alternatives, haurcaes. ob- jections apparently very serious have been brought forward. Against the physiological character of the action it has been urged (a) that wood, being supposed to be dead, could take no part in the ascent. of sap. It is known moreover (4) that killing the roots with boiling water does not prevent the ascent of sap. And, lastly (c), in the well- known experiments of Strasburger it was found that strongly poisonous solutions can be carried to the tops of trees. From these facts it has been held to be proved that the ascent of sap cannot be dependent on the livingness of the tissues concerned. ~* .