NATURE 



561 



THURSDAY, OCTOBER 9, 1890. 



A NEW THEORY FOR THE SENSITIVE 



PLANT, 



Das reisleiiende Gewebesystem der Sinnpflanze. By Dr. 



G. Haberlandt. (Leipzig : W. Engelmann, 1890,) 

 T^HE present decade has been a very important one 

 *- from the point of view of a botanical revival in this 

 country. The seed sown in previous years by Thiselton 

 Dyer and others did not fall entirely on sterile soil, and 

 gradually a school of active workers has arisen, sometimes 

 described, for want of a better name, as " the younger 

 school of botanists." The individuals constituting this 

 " school," though few in numbers, have pursued widely 

 ^diverging lines of research ; some devoting themselves to 

 jorphology, others to physiology and minute anatomy, 

 ^others again to the diseases of plants, &c. Important 

 results have accrued from their labours in the various 

 branches taken up, but in no case have they been more 

 striking than in the field of minute cell anatomy. The 

 readers of this journal hardly need to be reminded that 

 the discovery of the continuity of the protoplasm from 

 cell to cell, and the demonstration of the fact that plant 

 tissues do not consist of a number of isolated masses of 

 protoplasm, cut ofif from one another by the dead cell- 

 membranes, was largely due to the investigations of 

 Gardiner. 



The knowledge of the existence of these uniting fila- 

 ments seemed froni the first to throw light on many intri- 

 cate and obscure physiological problems. Foremost among 

 these was the possibility that by their instrumentality 

 the transmission of stimuli over considerable tracts might 

 be facilitated. It is interesting to remember that almost 

 the first case of continuity of protoplasm demonstrated 

 by Gardiner was that in the pulvini of the leaves and 

 leaflets of the sensitive plant. Nor did this lose its 

 significance when it was later realized that such a 

 continuity was a very general, if not universal, phe- 

 nomenon in plant tissues. The view that the stimuli, 

 which undoubtedly travel considerable distances in most 

 of the plants endowed with irritable movements, are 

 transmitted in virtue ;of these exceedingly fine uniting 

 filaments is one very generally held by botanists in this 

 country, and finds expression in Vines's " Lectures on 

 the Physiology of Plants." Indeed, that this is so, in certain 

 cases, has been experimentally demonstrated. In view 

 of these circumstances, the book whose name heads this 

 review, dealing as it does entirely with the mechanism of 

 stimulus transmission, will be studied with interest, and \ 

 the more so from the fact that Dr. Haberlandt's j 

 " Physiologische Pflanzenanatomie " has done much 

 towards the elucidation of many of the facts of anatomy, j 

 Dr. Haberlandt here limits himself solely to the investiga- 

 tion of the means by which a stimulus, set up at some point < 

 in the sensitive plant {Mimosa pudica), is transmitted to I 

 a distance, promoting movements in regions far removed 

 from the point stiniulated. 



During this century various physiologists have busied 



themselves with this problem, notably Dutrochet, Pfeffer, 



and Sachs. The prevalent theory on the Continent, which 



is associated especially with the name of Pfeffer, briefly 



NO. 1093, VOL. 42] 



amounts to this : — When an irritable portion of one of the 

 pulvini of the sensitive plant is stimulated, the irritable 

 cells lose their turgidity, water passing out of them into 

 the intercellular spaces associated with them ; a certain 

 amount of this water is said to enter the tracheides and 

 vessels of the xylem of the vascular bundle, and to upset 

 the hydrostatic equilibrium obtaining there ; this dis- 

 turbance is transmitted to a distance as a wave in the 

 I xylem, and stimulates, as it travels along, the irritable cells 

 I of the successive pulvini which it passes near, causing 

 them likewise to contract. This may affect merely the 

 pinnules of a single leaf, or, in certain cases, the stimulus 

 may travel from one leaf to another. 

 ! This hypothesis is based on very old experiments per- 

 formed by Dutrochet more than sixty years ago, Du- 

 \ trochet found that, (i) after the removal of a complete 

 ; ring or zone of cortical tissue from the stem, a stimulus 

 ; could still be propagated from one leaf to another, across 

 the decorticated region. A similar result followed when 

 the pith was destroyed, the vascular bundle alone being 

 I left intact. The irresistible inference was that the 

 j stimulus travelled by the vascular bundle. Further, (2) 

 j when the woody portion of the bundle was cut into, a 

 drop of liquid was observed to exude immediately, and a 

 j stimulus was transmitted upwards and downwards from 

 the point of lesion, causing movements in the nearest 

 leaves and even travelling to more distant ones. The 

 drop which exuded was supposed to come from the wood, 

 and the disturbance of pressure resulting, to initiate the 

 stimulus. Haberlandt deals with this " fundamental ex- 

 periment," and shows that Dutrochet and the others were 

 in error. To make this clear, it is necessary to briefly 

 indicate the structure of a bundle and adjacent tissues in 

 Mimosa pudica. In a transverse section of a stem (and 

 the same holds generally for the petiole) there is ex- 

 ternally the epidermis, below which comes the parenchy- 

 matous cortex. The cortex passes over into a zone of 

 thick-walled cells, described as collenchyma by Haber- 

 landt, as bast-fibres by some other writers. Within this 

 thick- walled zone is a ring of phloem, and finally the xylems 

 and pith. Dutrochet, when he thought he had dissected 

 away all the tissues outside the xylem in the experiment 

 recorded above (i), had, in point of fact, left not only the 

 phloem, but also the coUenchyma-ring intact. His knife 

 had been arrested by the collenchyma, which he had 

 mistaken for the wood. The collenchyma and phloem 

 remained intact, and the inference that the stimulus tra- 

 velled by the xylem was consequently a false one. 



Further, in the case of experiment (2), Haberlandt is at 

 great pains to show that the drops of liquid do not issue 

 from the wood at all, and establishes the fact that they 

 arise in reality from special cells in the phloem. Follow- 

 ing the same method of experiment as Dutrochet, Meyen, 

 and Pfeffer, Haberlandt demonstrates clearly — 



(i) That the stimulus normally travels inside the 

 collenchyma ring, but outside the xylem of the bundles ; 

 in other words, in the phloem. 



(2) That, when a stem is cut through, drops exude at 

 the moment of cutting. These drops arise, not from the 

 xylem, but from special cells in the phloem. 



This alone marks a considerable advance on the older 

 hypothesis. 



B B 



