THE PLANT 77 



the root), and so permitting of their growth. When the 

 plumule reaches the surface, and on exposure to light becomes 

 green in colour from the formation of chlorophyll, it is then 

 capable of assimilating carbon dioxide. The radicle soon 

 develops root-hairs, through which mineral substances and 

 nitrates, if present in the solution outside, can enter the 

 plant. 



The main parts of a plant are the roots, the stem, the leaves, 

 the flowers, and the seeds. 



The Roots. — The radicle, growing from the seed, at first 

 extends vertically downwards, its direction being determined 

 by gravitation or other force acting upon it. It afterwards 

 branches and sends out lateral roots. Near every growing 

 part of the root thin-walled root-hairs extend among the 

 particles of soil. As the root thickens the root-hairs die off; 

 they are only found in abundance near the growing ends of 

 roots. The thin walls of the root-hairs probably play a most 

 important part in the growth of the plant. In order to under- 

 stand their action it is necessary that some knowledge should 

 be possessed of the peculiar phenomena of diffusion and osmotic 

 pressure. 



Some reference to the former has already been made (see 

 chap. iii.). Colloidalj or glue-like substances diffuse very 

 slowly when dissolved in water, and have no power of diffusing 

 through insoluble colloidal bodies — e.g.^ parchment — while 

 crystallisable bodies diffuse rapidly, and can readily penetrate 

 a colloidal membrane if it be saturated with water. This 

 passage of dissolved crystalloids, as they are called, through a 

 colloidal membrane is a process of diffusion, and will always 

 take place from a stronger to a weaker solution, apparently 

 (but only apparently) ceasing when the solutions on either 

 side are equal in concentration. 



Certain substances, when arranged as a partition between 

 two solutions of different concentration, permit of the passage 

 of the solvent only, and prevent that of the dissolved substance. 



