44 



BOTANY: PRINCIPLES AND PROBLEMS 



The Plant Cell. — The root-hair (inchiding its basal portion) is a 

 plant cell. Since a knowledge of the structure and functions of 

 cells is obviously essential if we are to understand how the root- 

 hair is constructed and does its work, or, indeed, how any other 

 part of the plant is put together and functions, it will be necessary 

 at this point, bofoio wo discuss the physiology of the root, to 



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Aim. 



Fig. 28. — Diagrammatic section through a portion of a young root and the 

 adjacent soil, showing two root-hairs forcing their way between the soil particles. 

 Rock particles, water films, and air spaces in the soil are .shown. Much enlarged. 



describe briefly some of the more important characteristics of 

 cells in general and of plant cells in particular. 



We have already spoken of that remarkable living material 

 which is called protoplasm, the seat of all the various activities 

 which are maintained in animals and plants, and the only portion 

 of their bodies which is truly alive. Physically, protoplasm is 

 a thin, jelly-like, colloidal substance, but its minute structure is 

 not clearly known. Chemically, it is a mixture of very complex 

 proteins and is thus composed chiefly of carbon, oxygen, hydrogen, 

 and nitrogen. Protoplasm is the "physical basis" of all life and 

 the most extraordinary material known to man. 



The protoplasm of the plant body is not a directly continuous 

 mass but is broken up into minute parts, the cells or protoplasts, 

 each of which is a distinct and more or less independent unit, 

 possessing a definite structure and carrying on within itself a 

 variety of physiological processes (Fig. 27). Around the cell the 

 protoplasm secretes a dead cell-wall composed of the characteristic 



