70 



APPLIED BIOLOGY 



FIG. 27. Longitu- 

 dinal section of tip 

 of barley root, 

 showing root -cap 

 which covers the 

 rounded end of the 

 root. 



become stained, and in each cell a small rounded body will stain 

 brighter than the surrounding protoplasm. This body is the nucleus, 

 a very important part of the living matter of the 

 cell. Each cell has a cell-wall, which is the hard 

 substance of which wood is composed. Some 

 cells appear to have only a cell-wall and to be 

 filled with water or air. Such cells are older and 

 have lost the living matter which originally was 

 contained within their cell-walls. 



The above study of a root calls attention 

 to the fact that plants, like animals ( 39), 

 are composed of units called cells, and that 

 the essential living substance in cells is the 

 protoplasm. Plant cells multiply by di- 

 vision similar to that shown for animal 

 cells in Fig. 22. Epidermis stripped from 

 a stem of Tradescantia, and from the leaves 

 of an onion bulb, should be examined and 



compared with the epidermis of a frog ( 39). See also Figs. 



40 and 41. 



The Bean Stem 



69. General Structure of Stem. (L) Note the general shape 

 of the stem and its branches in a bean plant three weeks old 

 and in another six weeks old. The places where leaves are 

 attached are joints or nodes of the stem, and the parts of the 

 stem between the nodes are internodes. Compared with the hard 

 and woody stems of shrubs and trees, the stem of the bean 

 plant is soft (herbaceous), and soon after the formation of the 

 pods (fruit) the plant begins to wither and soon dies. Hence the 

 duration of life of a bean plant is one growing season; and such 

 a plant which does not naturally live over winter is an annual. 

 Name five other annuals. 



With a sharp knife, cut into a piece of a bean stem from a full- 

 grown plant and then peel off the soft rind (also sometimes 

 called cortex or cortical layer). Beneath the rind lies the hard 

 wood (xylem), which constitutes the central axis of the stem. Be- 

 tween the rind and the wood is a layer of very soft slippery substance 

 which makes it easy to separate rind from the wood. In early spring 



