1 80 THE LIFE OF THE PLANT 



the circular cut in the bark, convey the sap to the 

 lower part of the stem. Thus all these four obvious 

 experiments with willow and oleander branches gradu- 

 ally and sj^stematically reduce the circle of possible 

 suggestions, and in the end point decidedly to the 

 sieve-tubes as the course along which the nutrient sub- 

 stances of the plant spread — the plastic substances, so 

 called because they serve the purpose of building up 

 new parts in the plant. 



Recent investigations as to the distribution of the 

 latex-tubes in a leaf lend a certain colour to the suggestion 

 that they likewise serve as a very convenient course for 

 the movement of the nutrient sap. This suggestion is 

 based upon the fact that they are generally found in the 

 immediate neighbourhood of the green tissue of the leaf 

 where nutrient substances are manufactured. This 

 suggestion is supported by the observation that the loss 

 of latex exhausts some plants. 



After having traced the course of the movement of 

 the sap from the leaves, we have still to find out the 

 causes which set it in motion. Again, for the last time, 

 the key to the puzzle is found in diffusion — the word 

 which like a constant refrain is repeated every time 

 there is any question as to the absorption or transloca- 

 tion of matter into the plant from the external medium, 

 or from one part of the plant to another. According to 

 the laws of diffusion, matter when dissolved apparently 

 flows especially to places where it is changed into an 

 insoluble form, either being deposited in store for the 

 future, or simply spent in the building up of the solid 

 parts of the plant.^ The nutrient substances are 

 deposited all along the system of vascular bundles. 

 Cells surrounding these bundles are generally very rich 

 in starch, and occasionally also in crystals and other 

 matter. We have observed the storage of nutrient 

 substances in the endosperm of the seed ; similar 



' See chapters ii. and iii. 



