398 PLANT RESPONSE 



Excretion of nectar. — It is often assumed that the 

 excretion of nectar is due to plasmolytic action. The 

 excreted solution of sugar dries up by evaporation, and this 

 by plasmolysis draws up more from behind. We have seen 

 that the ascent of sap in the plant is brought about mainly 

 by rhythmic activity, and that concentrated solutions in the 

 leaves may help this movement osmotically. In the case of 

 nectaries, the presence of concentrated sugar solution outside 

 may thus help continued excretion, but this does not explain 

 the initiation of excretion, which could only have been caused 

 by the rhythmic activity of cells. 



Translocation of organic food-substances. — Though 

 the flow of organic food-materials towards places where there 

 is a deficit may be brought about by diosmosis, yet such a 

 mode of diffusion must be extremely slow. A more rapid 

 transport than could be produced by this means would appear 

 to be a necessity. There are certain considerations moreover 

 which may be brought forward, tending to show that this 

 translocation of food- materials receives considerable aid from 

 stimulatory actions. Czapek's observation, too, that there is a 

 cessation of translocation in a chloroformed leaf-stalk, points 

 to the inference that it is a physiological process. 



We know that an excitatory action proceeds from the 

 more to the less stimulated. Now the accumulation of a 

 large quantity of organic food-material may of itself act as a 

 stimulating agent in a given case ; thus an excitatory move- 

 ment would proceed from cell to cell, from places where there 

 was excess to places where there was deficit. In this way 

 large quantities of food-materials may be rapidly transported 

 by excitatory reaction, through conducting channels. And 

 such transport is also possible by stimulatory action even 

 through unspecialised cells. These ordinary cells are known 

 to be connected with each other, by means of pores and 

 plasmic threads. Mr. Horace Brown has shown {Phil. Trans. 

 vol. cxciii.) that transport of fluid may take place through 

 such a 'multi-perforate septum ' with almost as great rapidity 



