HOW THE PLANT LIVES 113 



nomena, and certain other conditions to which 

 growth is sensitive. 



2. The Factors of Growth 

 2a. Water in the plant 



184. The rigidity or stiffness of any herb 

 or succulent part is largely dependent on its 

 water content. If a succulent branch is severed, 

 it soon loses its water by evaporation, and it 

 becomes flaccid, or wilted. The proper exten- 

 sion, or turgidity, of the cells of plants with 

 water is necessary for active growth. The pas- 

 sage of the soil water into the plant, and there- 

 after its transfer from living cell to living cell, 

 is accomplished by the process of osmosis, which 

 is the diffusion of liquids through membranes. 

 Much of this water eventually reaches certain 

 conducting parts, or bundles. 



185. Surrounding each rootlet for some dis- 

 tance back of the tip is an enveloping growth of 

 delicate root-hairs. These hairs are single, 

 tubular cells, the outgrowth of single cells in 

 the outer layer of the root. Each one contains 

 within its walls, as do all active cells, living 

 matter called protoplasm, along with cell-sap. 

 In the soil these delicate hairs push readily in 

 amongst the soil particles, covering an immense 



