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or oval bodies, which are either loosely thrown together 

 (Fig. 103), or are pressed into a more or less compact 



Fig. 1C3. Fig. 104. Fig. 164 fa;. 



mass. In the latter case, owing to mutual pressure 

 they assume a somewhat angular form. These bodies 

 are called cells. They are hollow, and their walls are 

 usually thin and transparent. The entire fabric of 

 every plant, without any exception whatever, is made 

 up of cells; but as we proceed in our investigation, we 

 shall find that these cells are not all precisely alike, 

 tliat as they become older they tend, as a rule, to 

 thicken their walls and undergo changes in form, which, 

 ti a great extent, determii;e the texture of the plant's 

 . iibstance. 



1G2. A fabric made up of cells is called a tissue. A 

 collection of such cells as we found constituting our 

 pulp, and as we should find constituting the mass of 

 all the soft and new parts of plants, as well as of some 

 hard parts, is called cdluhir tissuv. Tlie cells com- 

 posing cellular tissue vary a great deal in size in different 

 l)lants, being, as a rule, largest in aquatics, in which 

 they may sometimes be observed with the naked eye. 

 Ordinarily, however, they are so minute that millions 

 of them iind room in a cubic inch of tissue. 



1G3. When young, the walls of the cells are quite 

 unbroken. Each cell is lined with an extremely thin 

 membrane, and a portion of its cavity is occupied by a 



