70 PLANT CELLS 



Chitin, a nitrogenous substance common in the exoskeleton of insects, is 

 a constituent of the walls of manj' fungi and bacteria. It has been reported 

 as being present in the walls of certain algae but it is unknown in any of the 

 higher plants. 



Tannins (Chap. XXII) are commonly found in the cell sap but they also 

 occur in the walls of certain tissues, especially cork and wood cells. 



JMucilngcs (Chap. XXII) are common constituents of the outer walls of 

 many water plants and occur also in the outer walls of some seed coats, in 

 glandular hairs and in other specialized tissues. 



Inorganic compounds such as silica and salts of calcium, iron, and other 

 metals are also present in some plant cell Avails. None of these inorganic 

 compounds, however, are regarded as essential constituents of the cell wall. 



Protoplasm. — In most mature cells of the vascular plants protoplasm 

 is present only as a thin layer covering the inner surface of the cell walls 

 but in some specialized cells branching strands of protoplasm also extend 

 across the vacuole. Under high magnification the protoplasm of active cells 

 appears as a colorless fluid, in which are suspended numerous tiny granules 

 and droplets of insoluble materials. These granules frequently exhibit active 

 Brownian movement. The fluid component of protoplasm also is frequently 

 in motion, streaming around the inner surfaces of the cell walls. Embedded 

 in this fluid component, and carried passively by it, are the specialized bodies 

 known as plastids. 



Although protoplasm appears to be a simple liquid, no simple liquid could 

 possibly possess the remarkable powers of synthesis, assimilation, reproduction, 

 growth and sensitivity that characterize the protoplasm of living plant cells. 

 The properties and behavior of protoplasm clearly show that it is not a sub- 

 stance but that it must be regarded as a complex system of substances. This 

 system is dynamic; it is constantly undergoing changes yet at the same time 

 the changes are so regulated and controlled that the system is not disrupted. 

 A cell is alive only so long as the organization of this dynamic protoplasmic 

 system is maintained. 



The protoplasmic system of living plant cells almost invariably contains a 

 well differentiated globular body known as the nucleus. All of the protoplasm 

 outside of the nucleus is designated as cytoplasm. Although the nucleus is 

 separated structurally from the cytoplasm by a delicate membrane the two 

 components are not physiologically isolated. The nucleus and the cytoplasm 

 appear to be mutually dependent and there is good reason to believe that the 

 nucleus controls and regulates the physiological processes that occur in the 

 cytoplasm. Probably because of the close relationship between the nucleus 

 and the cytoplasm many biologists use the terms "protoplasm" and "cyto- 



