86 



The Living Plant 



in order to grow, and, with the exception of a few which possess 

 long air passages connecting with the leaves, they take the in- 

 dispensable oxygen from air in the soil, by a method to be later 

 explained. A soil in the best condition for the respiration of roots 

 has the structure represented, under large magnification, in the 

 accompanying picture (figure 30). Soil is formed of particles 



Fig. 30. — A generalized drawing of a section, highly magnified, through a well-conditioned 

 soil and a fragment of root. The soil particles are dotted, the water is concentrically- 

 lined, the air spaces are left blank; into the soil project the root-hairs from the root 

 on the left. (Improved from a picture in Sachs' Lectures.) 



of rock, irregular in size and form. Around these particles and 

 in the angles between them is water, held in the capillary state, 

 while bubbles of air exist in the larger of the spaces among the soil 

 particles. When more water is added, then the air, being lighter, 

 is driven upwards and comes bubbling out of the ground; but 

 it returns again as the surplus water drains or evaporates away. 

 It is from this air in the soil that roots take their oxygen, and if 

 the air is kept out of the soil by excess of water, then the roots are 

 suffocated and die, precisely as air-breathing animals do when they 



