186 THE PLANT CELL. 



balance (turgidity) in the cells of the pulvini (small cellular 

 cushions), situated at the bases of the leaves and leaflets, whereBy 

 turgidity is lessened. The stimulus in this case passes along 

 certain cells surrounding the vascular bundles, and travels by 

 means of cytoplasmic connecting " bridles " between adjacent cells. 



Chemical substances exert at times a powerful attractive 

 influence upon protoplasm ; and, on the other hand, they may 

 repel the living substance. The growth of pollen-tubes is 

 brought about by such stimuli, as also is the attraction of anther- 

 ozooids towards the oospheres of Pteridophyta (see supra). In 

 the latter instance, enzymes often seem to be the chemical 

 substance producing the attraction, the process being known as 

 positive chemotaxis. Negative chemotaxis is seen in the repul- 

 sion of Jitlialium, produced by strong salt solution or acetic 

 acid. The selective action exercised ^>y the ectoplasm of root- 

 hairs is also of a kindred nature. 



The influence of moisture (hydrotropism) is sufficient at times 

 to determine the direction of growth of an organ (root). The 

 stimulus is in the main of a chemical nature. 



These few remarks on the chemistry and physiology of the 

 cell may serve to emphasise the fact that the c}top)asm is, as 

 was incidentally mentioned in Chapter i., capable of responding 

 to a variety of stimuli ; or, as is often said, possesses the pro- 

 perty of irritability. It has also been seen that protoplasm is 

 capable of transmitting stimuli from one cell to the cells of 

 remote parts ; and, finally, evidence has been put forward 

 showing that growth, as a whole, is the result of the action 

 upon the cytoplasm of the various physical and chemical agencies 

 v/hich are from time to time brought to bear upon it,* 



viii. — The Production of Heat, Light, and Changes of Electrical 

 Potential in a Cell, and the Action of Electric Currents 

 upon the Cytoplasm. 



The subject of heat-production in the cells of plants is one 

 which, from the experimental point of view, is often beset with 

 difficulties, owing to the inability to measure such small changes 



* Huxley long ago maintained that the vital properties of the proto- 

 plasm were the result of the disposition of the molecules of which it is 

 composed, and that no such term as " inherent vitality "' was necessary in 

 escribing the attributes of the living substance. (Mtthod and Eesults, 1S9,3.) 



