562 



NATURE 



[October 9, 1890 



Having cleared the ground so far, Haberlandt sets 

 himself two problems for solution, and it is with these 

 that the greater portion of his paper is occupied. These 

 questions are : — 



(i) What are the special organs in the phloem which 

 transmit the stimulus ? 



(2) What are the details of the mecharasm of trans- 

 mission ? 



As his later inferences and experiments are based on a 

 detailed anatomical study of the sensitive plant, it will be 

 advisable to follow Haberlandt into this matter. The 

 phloem of many Leguminosae is characterized by the pos- 

 session of rows of cells, somewhat larger than the true 

 sieve-tubes, which, from the nature of their contents, are 

 known as tannin-sacs. These, in Mimosa pudica, are 

 long, tube-like cells, arranged end to end. Each cell 

 possesses a primordial utricle and a large nucleus. The 

 longitudinal walls of these cells are frequently pitted, but 

 the structure of their transverse walls is characteristic. 

 Each possesses a very large, shallow pit. The closing- 

 membrane of the pit is traversed by a number of very 

 delicate protoplasmic filaments. These are much finer 

 than the similar connecting filaments of sieve-tubes, and 

 approximate more nearly to the uniting filaments of ad- 

 jacent parenchyma cells. Haberlandt shows that it is 

 a portion of the watery content of these cells which 

 escapes, when a stem or petiole is cut into ; and it is his 

 view that these cells constitute the organs which transmit 

 the stimuli from one point to another in the plant. The 

 watery fluid which exudes from them, usually clear and 

 colourless, gives a very characteristic, and deep reddish- 

 violet colour, with iron chloride, and, if allowed to dry 

 upon a slide, crystallizes out in various forms, usually 

 arranged as sphere crystals or dendritically. This sub- 

 stance is probably of the nature of a glucoside, since, 

 among other reactions, when treated with acids it is 

 broken up into glucose and a resinous body. Accom- 

 panying this glucoside is a considerable amount of 

 mucilage. 



The distribution of the vascular bundles, in especial 

 relation to these supposed conducting cells, both in leaf 

 and stem, is followed out in detail. The glucoside-con- 

 taining cells occur, roughly speaking, in two rings in the 

 phloem, one of which is nearer the collenchyma zone, the 

 other nearer the xylem. Some of the former actually 

 touch the collenchyma cells. Where the bundles traverse 

 the pulvini, a much larger proportion of these cells are 

 in contact with the collenchyma. In the leaflets all the 

 larger bundles are accompanied by the glucoside-con- 

 taining cells, but in the very small anastomoses they die 

 out. Finally, as to the distribution of protoplasmic con- 

 tinuity. This obtains in the soft cortex and in the 

 collenchyma (whose cells are freely pitted). The cells of 

 these two tissue systems are united together by ex- 

 tremely fine filaments in such a manner that an unbroken 

 protoplasmic continuity exists, from the periphery of the 

 pulvinus to the inmost layer of the collenchyma. In the 

 phloem also there exists a similar continuity. Between 

 these two systems, however— the soft cortex and collen- 

 chyma on the one hand, and the phloem (including the 

 glucoside cells) on the other — there is, according to 

 Haberlandt, no direct continuity ; and although the col- 

 NO. 1093, VOL. 42] 



lenchyma cells are freely pitted on all sides, the closing- 

 membranes of these pits are untraversed by protoplasmic 

 filaments on the side directed towards the phloem (and 

 glucoside cells). 



By careful experiments, referred to above, Haberlandt 

 demonstrates (what had been regarded by several ob- 

 servers as probable) that the stimulus travels in the 

 phloem ; and in view of the fact that the glucoside sacs 

 emit the drops of liquid on cutting a stem or petiole 

 (thus giving rise to a hydrostatic disturbance), and in 

 view also of inferences drawn from further experiments, 

 to be considered immediately, Haberlandt concludes that 

 the stimuli are transmitted by the rows of glucoside-con- 

 taining sacs, and this in a purely mechanical manner. Before 

 going on to elaborate his theory, Haberlandt meets and 

 disposes of the hypothesis, mentioned at the commence- 

 ment of this review, that the stimuli travel from cell to 

 cell by the agency of the uniting filaments of living 

 protoplasm. According to that view, when any pulvinus 

 is stimulated, as by a mechanical shock, its irritable 

 cells contract, losing their turgidity, and a movement 

 results ; at the same time the stimulus would be con- 

 veyed to the phloem, and there transmitted from cell to 

 cell by the filaments of protoplasm (it is immaterial 

 whether in the phloem-parenchyma, or even in the longi- 

 tudinally-running series of glucoside sacs) until it reaches 

 another pulvinus, where it would be communicated in 

 the same way to the irritable cells there, and a further 

 movement would result, and so on. 



It is necessary at this juncture to explain, so as to make 

 what follows intelligible, that physiologists have availed 

 themselves of two entirely different methods of stimu- 

 lating the plant — firstly, by submitting a pulvinus to a 

 mechanical shock, without damage to its tissues ; and 

 secondly, by cutting the petiole or stem, and causing 

 actual lesion of the conducting tissues. In both cases 

 the stimulus is transmitted, but in the latter to a much 

 greater distance, the method being altogether a more 

 violent one and perhaps quite different in its effects. 

 Pfeffer was content to regard anything of the nature of a 

 " vital " hypothesis of stimulus-transmission as disproved, 

 from the fact that, even when he chloroformed a definite 

 portion of a petiole, a stimulus could still be transmitted 

 through the region subjected to the chloroform. Haber- 

 landt, however, points out that this result must not be 

 taken as final, since there is no proof that the internal 

 tissues had been really acted on by the ansesthetic, as 

 applied by Pfeffer. Further, Vines has pointed out that, 

 although chloroform deprives the irritable cells of the 

 pulvinus of their irritability (rendering them rigid), there 

 is no justification for the assumption that it likewise 

 deprives the protoplasm of the coiiductiitg cells of their 

 conductivity ; an objection the validity of which is ad- 

 mitted by Haberlandt. It was necessary, therefore, to 

 make a more crucial experiment to decide this point, and 

 this Haberlandt does by substituting an actual killing of 

 the protoplasm in a small portion of a petiole for a mere 

 chloroforming. This was done by steaming a confined 

 zone of a petiole. Under these conditions, the stimulus, 

 to be transmitted successfully, had to pass through a dead 

 region. If the stimulus could be shown to traverse this 

 region, the " vital " hypothesis would be untenable. The 



