CHAPTER VIII 

 THE TISSUE SYSTEM OF LEAVES 



In the preceding chapter, cells are described as unit structures embody- 

 ing several interrelated physical and chemical systems. Cells are con- 

 sidered as the physiological units of plants because they are the smallest 

 bits of protoplasm known to be capable both of independent existence 

 and of reproduction through division. In later chapters less highly organ- 

 ized cells of bacteria and certain algae are described. There are several 

 thousand species of plants that hve as single cells. In other thousands 

 the cells are aggregated in colonies in which the individuals cells are 

 more or less independent. In the larger and more familiar plants the mil- 

 lions and billions of cells of which they are composed remain not only 

 fii-mly attached but are to some extent mutually dependent. During the 

 development of these plants, systems of cells called tissues arise through 

 cell enlargement and differentiation. 



Cells and tissues. The cells of a given tissue may have a common 

 origin; all of them may be similar in position, shape, texture, or color; 

 or several different kinds may form a distinct structural complex. Never- 

 theless, the tissues of many common plants may be distinguished readily. 

 In this chapter we shall try to picture the tissues of a leaf, emphasizing 

 the fact that the leaf, or foliage organ, like other organs of a plant, is a 

 system of tissues, the cells of which are differentiated, but intimately 

 related, structurally and physiologically. 



Leaf bud development. Leaves develop from buds; they first become 

 visible through the microscope as small protuberances (primordia) in 

 the meristematic^ region near the growing apex of the stem (Fig. 32). 

 The cells of the primordium divide and continue to produce new cells — 

 all very similar in size and shape. The uniform brick-like cells of the leaf 



^ A primordium ( pi. primordia ) is the beginning, original, or rudimentary state of an 

 organ — in most cases, a minute mound of similar cells. The term meristem (Greek 

 meris = a divider) refers to any part of a plant in which cell division, enlargement, and 

 differentiation are possible or taking place; it often is a synonym of growing region. 

 Meristematic cells, tissues, and regions are sometimes called embryonic cells, tissues, and 

 regions. After cells and tissues are mature they may again become meristematic through 

 changes in their immediate enxironment, and cell division begins again. 



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