CHAPTER XXXIII 



TISSUES 



40G. The lowest plants are unicellular or composed of 

 only one cell. Of such are bacteria (Fig. 123). 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. 



407. Any group of similar cells is called a tissue. 

 Each of the different tissues of a plant lias its own type of 

 cells, although the cells in a tissue may differ from each 

 other in various minor ways. 



408. 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. 

 Parenchymatous tissue is found at the growing point 

 of a shoot or root (Fig. 395); in the mesophyll (soft 

 pulpy part) of the leaves (Fig. 411); around the vascular 

 bundles of stems and roots (Fig. 402/), and in a few other 

 places, as pith, medullary rays, etc. The cells of this tis- 

 sue may be meristematit in a state of active division and 

 growth ; or they may be permanent, no longer able to 

 divide. 



409. 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 point. 

 This differentiation takes place in the region just back of 



(252) 



