APPARATUS OF VEGETABLE PHYSIOLOGY. 263 



other having the rapidity of their growth diminished by 

 the action of light, that there are, in fact, cells which are 

 positively or negatively heliotropic, just as there are cells 

 which are positively or negatively geotropic. 



Growth is, however, only one of the processes of plant 

 life which are influenced by the mechanical action of the 

 more refrangible rays of the spectrum. For instance, the 

 movements of zoospores, 1 as well as those of the motile 

 parts of the higher plants, are all influenced by these rays ; 

 and we may say generally that the more purely physical 

 phenomena of plant life are especially affected by them. 



The less refrangible rays such as would penetrate into 

 the interior of this orange-coloured flask are concerned 

 with the chemical processes of plants. Of the truth of 

 this statement I am unable on this occasion to give you 

 any demonstration. I can, however, refer you to experi- 

 ments which show conclusively that some of the most 

 important chemical processes can only take place in the 

 presence of such rays. Assimilation, for instance that 

 process in which the carbonic acid gas present in the air 

 is decomposed by the green colouring matter of the leaves, 

 and the carbon thus obtained is combined with oxygen and 

 hydrogen derived from water to form starch, whilst at the 

 same time oxygen is liberated can only take place under 

 the influence of these rays of low refrangibility, the yellow 

 rays being particularly active in promoting this process 

 (Draper ; Pfeffer). 



With the apparatus of Prof. Sachs our list closes. 

 The instruments exhibited are few in number and are 

 comparatively simple in construction, but they are of 

 considerable interest, for the application of them has 

 materially contributed to the solution of some of the more 

 important and difiicult problems of Vegetable Physiology. 



1 Since the above was written Prof. Sachs has published a paper,' 

 in which he shows that the apparent heliotropism of zoospores is pro- 

 duced by currents in the water, in consequence of slight differences 

 of temperature in different parts. 



