666 REPORT — 1891. 



and reaction is fully given, and is applied, among other cases, to that of heliotropism 

 and geotropism. PJeff'er states clearly, and -without reserve or obscurity, the view 

 that light and gravitation act as stimuli or releasing forces, in manners decided by 

 the organisation of the plant. Pfeffer seems to me to be the first writer who has 

 treated the subject fully and consistently. 



In Sachs' ' Vorlesungen,' 1882, a view similar to that briefly sketched in his 

 paper of 1879 is upheld. Geotropism and heliotropism are described as Reiz- 

 erscheinungen, i.e. phenomena of stimulation. The phenomena in question are 

 described under the heading Anisotropy, a word which expresses, according to 

 Sachs,' ' the fact that different organs of a plant under the influence of the same 

 external forces assume the most varied directions of growth.' In another passage '" 

 he states that the anisotropy of the diflerent organs ' is nothing else than the ex- 

 pression of their diflerent irritability to the influence of gravity [and] light, &c.' 



Vines,^ who has recently (1886) summarised the evidence on growth curvatures, 

 and whose researches on kindred subjects entitle his opinion to respect, accepts 

 fully the view that gravitation, light, &e., act as stimuli. 



It is not necessary to trace the subject further, the views under discussion being- 

 now well recognised canons of vegetable physiology. 



I cannot, however, omit to mention Pl'efl'er's'' brilliant researches on the 

 cbemotaxis (irritability to certain reagents) of low organisms, such as antherozoids 

 and bacteria. To take a single instance, I'l'efl'er showed that the antherozoids in 

 responding to the effect of malic acid follow precisely the same law that in animals 

 correlates the strength of stimulus and amount of effect. This result, altliough it has 

 no direct connection with growth-curvatures, is nevertheless of tlie highest import- 

 ance in connection with the general question of vegetable irritability. 



Nor can I omit to mention the ingenious reasoning by which NolP localised 

 the seat of irritability in a vegetable cell. He points out how in acellular plants, 

 such as Caulerpa or Derbesia, the flowing protoplasm may travel from positively 

 geotropic root to apogeotropic stem, and he argues from this that the motile 

 endoplasm cannot be the seat of specific irritability. The flowing plasma 

 which is always changing its position with regard to external forces must be as 

 fully incapacitated from responding to them as though the plant were turning on 

 a klinostat. It follows from this that it must be the stationary ectoplasm which 

 perceives external change. From a different point of view this is what we should 

 expect — we should naturally suppose that the part which regulates the growth of 

 the membrane, and therefore the curvature of the cell, should be the irritable 

 constituent of the cell contents. 



In attempting to trace the history of the establishment of growth-curvatures as 

 phenomena of irritability, I have been forced to confine myself to a slight sketch. 

 I have found it impossible to give a full account of the course of research on the 

 subject. I have given an account of some of the halting-places in the journey of 

 thought, but not of the manner in which belief has travelled from stage to stage. 

 Far greater knowledge than mine would he required to compile such an itinerary. 



Meclianism. 



The first step m advance of Hofmeister's views was the establishment that the 

 curvatures under consideration are due to unequal growth, that is to say, to an 

 excess of longitudinal growth on the convex than the concave side. It is not, 

 however, easy to say how far Hofmeister had this idea, for it, in fact, depends on 

 how we define 'growth.' Hofmeister knew, of course, that the convex side of q 

 curved shoot was longer than it had been before the curvature occurred ; this is a 

 mathematical necessity. But he also made out the important point that the con- 

 cave side increases in length during the curvature. These permanent elongations he 

 must have known to be growth, but his attention was directed to what is, after all, 

 the more important point, namely, why it was that unequal elongation took place. 



Sachs, in his ' Experimental-Physiologic,' held that growth curvatures are due 



' P. 855. s P. 859. » Physiology of Plants. 



* Tubingen. Untersuchungen, vol. i. * Sachs' Arbeiten, vol. ii. p 466. 



