October 9, 1890J 



NATURE 



563 



result was most instructive. When an ordinary mechanical 

 stimulus was applied, there was no transmission ; when, 

 however, a stimulus, caused by a lesion of the tissues, 

 was applied, it was transmitted, in the majority of cases, 

 over the zone previously killed by steam. From this, 

 Haberlandt concludes that no theory which depends on | 

 filaments of protoplasm to conduct the stimulus can be 

 maintained. He considers it improbable that a me- 

 chanical stimulus and one due to lesion should travel in 

 different ways, notwithstanding the fact, which he men- 

 tions later on, that under special conditions a stimulus 

 can be transmitted even by the vessels and tracheides of 

 the xylem. There is nothing surprising in the trans- 

 mission over a dead region of a stimulus due to lesion. 

 It is just this sort of stimulus that would cause a con- 

 siderable upset in the hydrostatic equilibrium in the 

 glucoside cells cut into, and it is conceivable that the 

 disturbance due to this sudden fall in pressure might be 

 conveyed, in a purely mechanical manner, over consider- \ 

 able distances. Hitherto it has not been shown that a 

 normal 7nechanical stimulus can be transmitted over a 

 zone that has been rendered incontestably dead. 



If the distribution of the continuity of protoplasm in a 

 pulvinus should be, as stated by Haberlandt, such that 

 the outer system of continuous cells extends inwards only 

 so far as, and including, the collenchyma, and that the 

 phloem is independent of this system, then it is difficult 

 to see how the stimulus could pass (by a vital hypothesis) 

 from the irritable and contractile cells to the conduct- 

 ing cells. This result, however, requires confirmation. 

 Gardiner, who investigated the nature of the continuity 

 in these organs of the sensitive plant, makes no comment 

 on any such marked discontinuity of the protoplasm, and 

 the inference, drawn from a study of his paper in the 

 Philosophical Transactions, is that the cells, from the 

 periphery right up to dead vessels of the wood, consti- 

 tute one connected whole. What he denies to the sensitive 

 plant, Haberlandt admits for other cases. The pheno- 

 menon of transmission of stimuli in the stigmas of 

 Mimuhts and Martynia, in the leaves of Dioncea and 

 probably of Drosera, and in the tendrils of many climbing 

 plants, would seem to be a function of the protoplasmic 

 fibrils. Having dealt with his refutation of the "vital" 

 hypothesis, as applied to the sensitive plant, we may 

 give a short summary of the theory put forward by 

 Haberlandt, though for its details, and the many ques- 

 tions raised therein, the reader is referred to the original 

 text. Haberlandt holds that both in the case of a 

 mechanical stimulus, and in that of one caused by a lesion 

 •of the tissues, the transmission is effected in a purely 

 mechanical manner, as a wave or impulse passing along 

 the glucoside-containing cells. Necessarily the trans- 

 verse walls, which possess each a broad, shallow pit, are 

 regarded as offering little resistance to the filtration of 

 the contained sap. The protoplasm which is continuous 

 through these pit-membranes is not regarded as playing 

 any important part in this event. When a pulvinus is 

 mechanically stimulated, its irritable cells lose water and 

 contract; the disturbance set up by this fluctuation in 

 pressure will start a wave in the rows of conducting cells, 

 travelling from the point at which the increase of pressure 

 occurs. The wave is what may be described as a positive 

 NO. 1093, VOL. 42] 



wave {Bergwelle), and the method of its transmission is 

 similar to that obtaining in a closed rubber tube distended 

 with water, when it is pinched at one end. This wave, 

 when it reaches the irritable cells of the next pulvinus, 

 will be communicated to them through the elastic collen- 

 chyma layer, probably through the pits, which are 

 numerous. In more sluggish cases it may not, perhaps 

 be till an actual bendmg occurs in the stimulated pulvinus 

 that the increase of pressure will be sufficient to start the 

 wave in the conducting cells. 



On the other hand, when the stimulus is due to lesion, 

 as when a petiole or internode is cut into with a sharp 

 knife, the wave is set up in a different manner. At the 

 moment of cutting into the turgid, glucoside-containing 

 cells, a drop of liquid escapes, causing a fall in pressure. 

 This is transmitted as a negative wave {Thalwelle), and 

 will be communicated to the irritable cells at a distance, 

 by the agency of the pits there. In this case, however, 

 the pit-closing membranes of the collenchyma will bulge 

 slightly inwards, in the former case outwards. The com- 

 munication from the conducting to the contractile cells is 

 rendered easier from the fact that in the pulvinus a much 

 larger proportion of the former lie adjacent to the collen- 

 chyma than at other points on the course of the bundle. 

 This special arrangement undoubtedly seems to favour 

 such a theory as that of Haberlandt. Still it must be 

 remembered that it may be due to quite different causes. 

 As is well known, the arrangement of the bundles is 

 always considerably modified at the pulvini, so that the 

 bending may be interfered with as little as possible. 



In conclusion, it must be admitted that the paper here 

 reviewed is a masterly piece of work, though, it may 

 { be, many naturaHsts in this country will hardly agree 

 , with the author in the inferences which he draws, and 

 I the theory which he builds upon them. It must be borne 

 1 in mind that possibly too important a part may have been 

 assigned to the uniting fibrils of protoplasm as touching 

 the transfer of stimuli of various kinds from cell to cell. 

 I In the first blush of a great discovery, of so far-reaching 

 a nature as that of the continuity of protoplasm, and 

 more especially from the fact that at a very early period it 

 ', was in the sensitive pulvini that this continuity was shown 

 ! to exist, — in view of this, the position that has been taken 

 I up by workers in this country may have been an over- 

 1 sanguine one. It may be that the explanation of this 

 phenomenon of protoplasmic continuity (though un- 

 I doubtedly it facilitates transmission of stimuli in certain 

 ' cases, e.g. stigmas of Mimiilus, &c.) may have another 

 bearing — that it may in some way affect the nutrition of 

 the pit-closing membrane, or even discharge the purely 

 mechanical function of binding the protoplasts to the 

 closing membranes. For the present, although it must 

 be conceded that Haberlandt has considerably advanced 

 our knowledge of this question, in that he has localized 

 the conducting region to the phloem, and has shown that 

 stimuli due to actual lesion can be transmitted in a purely 

 j mechanical manner ; nevertheless he has failed to demon- 

 strate conclusively the untenability of the " vital " hypo- 

 ! thesis in the case of the normal stimulus {Siossreis). This 

 being so, further results must be awaited before this in- 

 teresting question can be regarded as finally settled. 



F. W. O. 



