2. THE STRUCTURE OF ECONOMIC PLANTS 



The Protoplast. — The protoplast is commonly differentiated 

 into several parts: cytoplasm, nucleus, plastids, and other cell 

 inclusions. 



The cytoplasm constitutes the main mass of the protoplast, and 

 at its limiting outer periphery is the plasma membrane. Physically, 

 cytoplasm is a colloidal system, probably of the emulsion type, 

 with properties similar to those of other recognized colloids such as 

 viscosity, adsorptive power, semi-permeability, and its structural 







«- 



Fig. I. Meristematic cells from the growing point of the root of Liliuin Harrisii showing 

 character of the cytoplasm and nuclear activity. 



reactions to external and internal factors both chemical and physi- 

 cal. Optically, cytoplasm appears to be a hyaline, homogeneous 

 semi-fluid with numerous inclusions (granules, globules of fat and 

 oil, chondriosomes, plastids, etc.) which occur in varying sizes and 

 frequencies. The physical appearance of the colloidal system may 

 be modified by the degree of viscosity of the cytoplasm as well as 

 by the extent and character of vacuolization. In a young meriste- 

 matic cell, the cytoplasm is relatively dense with numerous minute 

 vacuoles; but, as it matures, these become fewer and larger until 

 finally a large central vacuole or sap cavity is formed. In this case, 

 the protoplast is limited centripetally by a vacuolar membrane and 



