78 FOUNDATIONS OF BIOLOGY 



With this diagrammatic arrangement of the tissue systems 

 of the plant in mind, we are in a position to consider the 

 histology of a typical root, stem, and leaf of the higher 

 Flowering Plants; in other words, to resolve the cylinders or 

 tissue systems of the plant into their component parts by 

 the study of transverse and longitudinal sections cut at 

 various levels, and so to determine the general character 

 and distribution of the cells as seen under the microscope. 



1. Root 



An examination of the tip of a root shows that it is covered 

 with a large number of cells which form the ROOT CAP. 

 These cells are gradually rubbed away as the root works 

 through the soil and continually replaced by new ones from 

 the GROWING POINT which is immediately above. The nu- 

 merous, small, densely-packed cells constituting the growing 

 point represent the region of cell formation for the entire root 

 tip, since near the center is a group of cells from which 

 smaller cells are divided off, and these in turn absorb food 

 materials and attain the normal size. It will be recalled 

 that the growing point is continuous with the cambium 

 region above, and it thus represents the growth system 

 (meristem) at the root tip. (Fig. 43.) 



Just above the growing point is the GROWTH ZONE which 

 includes cells recently formed by the tip in its growth down- 

 ward. In this region the cells enlarge rapidly, especially in 

 length, and at the same time retain relatively thin cell walls. 

 The cytoplasm of these cells, by the development and coales- 

 cence of large vacuoles of cell sap (water, sugar, and other 

 substances in solution) , soon forms merely a lining closely ap- 

 plied to the wall ; a condition characteristic of many plant cells 

 in contrast with those of animals. In the growth zone also 

 is clearly seen on the surface the protective layer, or epidermis, 



