474 



SCIENCE. 



[N. S. Vol. XII. No. 300. 



vinus of Mimosa pudica is alone sensitive to 

 contact was ascertained by Burnett and 

 Mayo in 1827; and shortly after (1834) 

 Curtis discovered the sensitiveness of the 

 hairs on the upper surface of the leaf of 

 Dioncea. After a long period of neglect the 

 subject was taken up by Darwin. The ir- 

 ritability of tendrils to contact had been 

 discovered by Mohl in 1827 ; but it was 

 Darwin who ascertained,. in 1865, that it is 

 confined to the concavity near the tip. In 

 1875 Darwin found that the irritability of 

 the tentacles of Drosera is localized in the 

 terminal gland ; and followed this up, in 

 1880, by asserting that the sensitiveness of 

 the root is localized in the tip, which acts 

 like a brain. This assertion led to a great 

 deal of controversy, but the researches of 

 Pfeffer and Czapek (1894) have finally es- 

 tablished the correctness of Darwin's con- 

 clusion. It is interesting to recall that 

 Erasmus Darwin had suggested the possible 

 existence of a brain in plants in his ' Phj'- 

 tologia' (1800). But the word 'brain' is 

 misleading, inasmuch as it might imply 

 sensation and consciousness : it would be 

 more accurate to speak of centers of gan. 

 glionic activity. However, the fact remains 

 that there exist in plants irritable centers 

 which not only receive stimuli but transmit 

 impulses to those parts by which the conse- 

 quent movement is effected. The transmis- 

 sion of stimuli has been found in the case 

 of Mimosa pudica to be due to the propaga- 

 tion of a disturbance of hydrostatic equilib- 

 rium along a special tissue ; in other cases, 

 where the distance to be traversed is small, 

 it is probably effected by means of that con- 

 tinuity of the protoplasm to which I have 

 already alluded. 



Finally, as regards the mechanism of 

 these movements, we find Senebier and Eu- 

 dolphi, the earliest writers on the subject 

 in the nineteenth century, asserting, as if 

 against some accepted view, that there is 

 no structure in a plant comparable with 



the muscle of an animal. Eudolphi (1807) 

 suggested, as an alternative, that the posi- 

 tion of a mobile leaf is determined by the 

 ' turgor vitalis ' of the pulvinus, and thus 

 anticipated the modern theory of the mech- 

 anism. But he gives no explanation of 

 what he means by ' turgor '; and the term 

 is frequently used by writers in the first 

 half of the century in the same vague way. 

 Some progress was made in consequence of 

 the discovery of osmosis by Dutrochet 

 (1828), and more especially by his observa- 

 tion (1837) that the movements of Mimosa 

 aredependent on the presence of oxygen, and 

 are therefore vital. But it was not, and 

 could not be, until the existence of living 

 protoplasm in the cells of plants was real- 

 ized, and the movements of free-swimming 

 organisms and naked reproductive cells had 

 become more familiar, that the true nature 

 of the mechanism began to be understood; 

 and then we find Cohn saying, as long ago 

 as 1860, that ' the living protoplasmic sub- 

 stance is the essentially contractile portion 

 of the cell.' This statement may, perhaps, 

 seem to put the case too bluntly and savor 

 too much of animal analogy ; but the study 

 of the conditions of turgidity has shown 

 more and more clearly that the protoplasm 

 is the predominant factor. The protoplasm 

 of plant-cells is undoubtedly capable of 

 rapid molecular changes, which alter its 

 physical properties, more particularly its 

 permeability to the cell-sap. It may be 

 that these changes cannot be directly com- 

 pared with those going on in animal muscle; 

 but if we use the term ' contractility ' in its 

 wider sense, as indicating a general prop- 

 erty of which muscular contraction is a 

 special case, then Cohn's statement is fully 

 justified. This is borne out by the obser- 

 vations of Sir J. Burdon-Sanderson (1882- 

 88) on the electrical changes taking place 

 in the stimulated leaf of Dioncea, and by 

 Kunkel's (1878) corresponding observations 

 on Mimosa puhlica : in both cases the electri- 



