CHAPTER II 

 ELEMENTARY TISSUES 



Variation in Cells. Cells vary greatly in size, shape, thick- 

 ness of cell wall, markings of cell wall and in contents. These 

 modifications can all be traced from the simplest and most primi- 

 tive of cells. The cells of the lower plants are very simple and 

 very similar in structure, chemical composition and functions, 

 but in succeeding higher groups we find gradual changes in 

 character involving all these points. In the young embryo of 

 the higher plants we find the cells are the same in structure, 

 chemical composition and function ; but with the development of 

 the embryo and seedling we find the formation of new types 

 of cells which perform special functions. The complexity of the 

 plant varies primarily with its position in the plant kingdom. 

 These variations and groupings of cells give rise to what are 

 known as tissues. 



A tissue, therefore, may be defined as a group of cells similar 

 in origin, structure and functions. All plants, regardless of 

 complexity, begin as single cells, and the great variety of cells, 

 tissues, systems and organs hereafter described are derived from 

 these individual cells as a result of growth, cell division and cell 

 modification. This gradual modification of a cell or group of 

 cells for the performance of a particular function is known as 

 differentiation and results in the formation of a number of ele- 

 mentary tissues, which will be described in this chapter. 



Parenchyma (Fig. Y). This tissue is composed usually of 

 thin-walled cells, which are generally large and loosely attached, 

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