CHAPTER XXXVIII 



TISSUES 



469. The lowest plants are unicellular or composed 

 of only one cell. Of such are bacteria. (Fig. 136.) All the 

 higher plants are composed of collections or aggregations 

 of innumerable cells: they are multicellular. If we ex- 

 amine the cells of the stem, the leaves, and the roots of any 

 common garden plant we find that they differ very widely 

 from each other in shape, size, and texture. 



470. Any group of similar cells is called a tissue. Each 

 of the different tissues of a plant has its own type of cells, 

 although the cells in a tissue may differ from each other 

 in various minor ways. 



471. Parenchymatous Tissue. Thin-walled cells are 

 known as parenchyma cells. When they unite they form 

 parenchymatous tissue. These may or may not be elon- 

 gated in form, and they usually contain protoplasm. Paren- 

 chymatous tissue is found at the growing point of a shoot 

 or root (Fig. 448); in the mesophyll (soft pulpy part) of 

 the leaves (Fig. 468) ; around the vascular bundles of stems 

 and roots (Fig. 455f), and in a few other places, as pith, 

 medullary rays, etc. The cells of this tissue may be meriste- 

 matic in a state of active division and growth; or they may 

 be permanent, no longer able to divide. 



472. One important use of this tissue is to form other 

 tissues, as in growing points. Near the end of any young 

 root or shoot the cells are found to differ from each other 

 more or less, according to the distance from the ponit. 

 This differentiation takes place in the region just back of 

 the growing point. In the mesophyll (or middle soft part) of 



(278) 



