LECTURE IV. 



ABSORPTION. 



IN the first lecture we met with a very simple case of this 

 function. We found that the yeast-cell, floating in Pasteur's 

 solution, is capable of absorbing water and dissolved substances, 

 and it is in this way that all plants obtain the materials of 

 their food. It is characteristic of plants that they can only 

 absorb their food in the fluid form, for everything that they 

 take up has to pass through closed cell-walls. In the higher 

 plants absorption does not take place to an equal extent at 

 all points of their surface, as is apparently the case with 

 the Yeast-plant, but we find certain members of these plants 

 exhibiting special adaptation for the performance of this 

 function. These absorbent organs are the roots, as regards 

 water and substances in solution, and the leaves, as regards 

 gases. 



In thallophytic plants there are no roots or leaves : water and sub- 

 stances in solution are absorbed either directly by the cells of the thallus 

 or by root-hairs (rhizoids) ; gases are absorbed by the cells of the thallus. 

 The Muscineae also have no true roots, but only root-hairs. 



I. Absorption of Water and of Substances in Solution. 



The parts of the roots of the higher plants which are 

 active in absorption are the ropj^hairs and the uncuticularised 

 epidermal cells of the younger roots. The root-hairs of these 



