268 CRYPTOGAMS 



It is the physiological unit of the plant or animal body. The 

 plant or animal is the sum total of its cells. Every cell 

 consists of protoplasm and has a nucleus. Vegetable cells 

 usually also have a cell wall which is composed of a sub- 

 stance called cellulose, secreted by the protoplasm of the cell. 

 (See Ex. 147.) 



Every living cell can select, change, assimilate, grow, 

 excrete, and perform all other functions of protoplasm. 



They can reproduce other cells by a process of division 

 which can be seen in sections of young root tips and by bud- 

 ding, as we saw in yeast. 



Growth of plants is increase in size due to an increase in 

 the number of cells. 



Every living thing, whether plant or animal, begins its 

 existence as a single cell known as an egg. By a series of 

 cell divisions followed by corresponding growth the embryo 

 is at length developed and all later growth is due to increase 

 in number of the cells. 



Adaptation of parts to function applies to cells as well 

 as to plant organs. Thus are developed all manner of cell 

 forms, such as root hairs, guard cells, palisade cells, piths, 

 pollen, wood, etc., each adapted to its own peculiar 

 work. 



Tissues. When cells are engaged in doing the same sort 

 of work, they assume similar form and appearance. Such 

 a collection of similar ceils is called a tissue. Thus we 

 have seen epidermis, cortex, and fibrovascular tissues in the 

 root. In the stem were pith, bark, bast, spiral, and many 

 other tissues. In the leaf were epidermis, palisade, spongy 

 parenchyma, etc. In like manner, animals have bony, 

 muscular, nervous, fatty, and many other tissues. 



Organs. An organ is a tissue or collection of tissues 



