CHAPTER III 



THE DIFFERENTIATION OF PLANT-CELLS 



FOUR principal types of element l are found in vegetable tissues. 

 A large part of the plant-body is composed of cells which usually 

 appear rounded or polygonal in transverse sections, and in most 

 cases contain a living protoplast. In longitudinal sections they 

 are often rectangular in form, seldom more than two or three 

 times as long as broad, and provided with square or rounded 

 ends (cf. Fig. 9). Such cells, described as parenchymatous, 

 frequently have thin walls, and often form an extensive tissue 

 known as parenchyma. 



In contrast to this type are other cells, termed prosenchyma- 

 tous, in which the length greatly exceeds the breadth, such 

 elements being found more particularly in the mechanical and 

 conducting tissues. These cells usually have pointed ends and 

 thickened walls (Fig. 16, C, D), and the width, as seen in trans- 

 verse section, is commonly small as compared with that of a 

 parenchymatous element (Fig. 16, B). Occasionally forms of 

 cells are encountered which are transitional between these two 

 types. 



The elements of the third type are of quite a different char- 

 acter, and are termed vessels (Fig. 17). They are the principal 

 water-conducting structures in the wood of Flowering Plants, 

 possess no living contents when mature, and their longitudinal 

 walls are thickened in the various ways to be described below 



1 These, and the various modifications described in the present chapter, 

 are most easily studied by teasing out (i.e. tearing to pieces with a pair 

 of needles) small portions of a Celery-stalk or Marrow-stem that have 

 been previously boiled for some time in water ; the middle lamellae are 

 thereby dissolved, so that the individual cells readily separate from one 

 another. 



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