928 REPORT— IDOO. 



one more century haa closed without bringing tlie solution of the seculai* pl'obleifl 

 of the ascent of the sap. 



The nineteenth century has been, fortunately, rather more fertile in discovery 

 concerning the movements and irritability of plants. But it is surprising how much 

 knowledge on these points had been accumulated by the beginning of the century : 

 the facts of plant-movement, such as the curvatures due to the action of light, the 

 sleep-movements of leaves and flowers, the contact-movements of the leaves of the 

 sensitives, were all familiar. The nineteenth century opened, then, with a con- 

 sidnrable store of facts ; but what was lacking was an iuterpretation of them ; and 

 whilst it lias largely added to the store, its most important work has been done in 

 the direction of explanation. 



The first event of importance was the discovery by Knight, in 1806, of the fact 

 that the stems and roots of plants are irritable to the action of gravity and respond 

 to it by assuming definite directions of growth. Many years later the term 

 ' geotropism ' was introduced by Frank (1868) to designate the phenomena of 

 growth as aSected by gravity, and at the same time Frank announced the import- 

 ant discovery that dorsiventral members, such as leaves, behave quite differently 

 from radial members, such as stems and roots, in that they are diageotropic. 



It was a long time before the irritability of plants to the action of light was 

 recognised. Chiefly on the authority of de Candolle (to whom we owe the term 

 ' heliotropistn '), heliotropic curvature was accounted for by assuming that the one 

 side received less light than the other, and therefore grew the more rapidly. But 

 the researches of Sachs (1873) and MUller-Thurgau (1876) have made it clear that 

 the direction of the incident rays is the important point, and that a radial stem, 

 obliquely illuminated, is stimulated to curve until its long axis coincides with the 

 incident rays. Moreover, the discovery by Knight (1812) of negative heliotropism 

 in the tendrils of Vitis and Ampelopsis really put the Candollean theory quite out 

 of court ; and further evidence that heliotropic movements are a response to the 

 stimulus of the incident rays of fight is afibrded by Fi-ank's discovery of the 

 diaheliotropism of dorsiventral members. 



The question of the localisation of irritability has received a good deal of 

 attention. The fact that the under surface of the pulvinus 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 irritability 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 localised in the terminal gland ; and followed this up, in 1880, 

 by asserting that the sensitiveness of the root is localised 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 established 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 ' Phytologia ' (1800). But the word ' brain ' 

 is misleading, inasmuch as it might imply sensation and consciousness : it 

 would be more accurate to speak of centres of ganglionic activity. However, 

 the fact remains that there exist in plants irritable centres which not only 

 receive stimuli but transmit impulses to those parts by which the consequent 

 movement is efiected. The transmission of stimuli has been found in the case of 

 Mimosa pudica to be due to the propagation of a disturbance of hydrostatic 

 equilibrium along a special tissue ; in other cases, where the distance to be traversed 

 is small, it is probably effected by means of that continuity of the protoplasm to 

 ■which I have already alluded. 



Finally, as resrards the mechanism of these movements, we find SeniSbier and 

 Rudolphi, 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. Iludolphi (1807) suggested, as an alternative, that 

 the position of a mobile leaf is determined by the ' turgor vitalis' of the pulvinus, 

 and thus anticipated the modern theory of the mechanism. But he gives no 



