58 GENERAL BOTANY 



a cellular embryo. In the second phase of development the 

 uniform cells of the early cellular embryo begin to differentiate 

 to form the first organs and tissues of the early embryo. In the 

 third phase this early embryo forms the seedling, in which the 

 parts outlined in the early embryo take on the form of the parts 

 of the mature plant. 



The mature form and structure of the plant is derived from 

 the seedling by the growth and differentiation of cells located 

 in .particular growing regions, called meristems and cambiums, 

 at the tips of the stem, roots, and branches and in the internal 

 growing layers (cambiums) of the plant body. 



Physiology. The physiology of both plants and animals has 

 been shown to be dependent upon the physiology of the con- 

 stituent cells of the plant and animal body. Nerve cells and 

 their processes are known to receive and send impulses which 

 cause muscle cells to contract and move the parts of the an- 

 imal body. In a similar manner the cells of plants cause rapid 

 or slow movements in roots, stems, and leaves. In the sensitive 

 plant (Mimosa) the movements caused by specialized cells at the 

 base of the leaves and leaflets are as rapid as those of some 

 animals, but the slower movements of the parts of plants have 

 been shown to be no less cellular in their mechanism than those 

 of Mimosa and in animals. Similarly, food-building and the cir- 

 culation and use of foods are ultimately traceable to the activi- 

 ties of the constituent cells of plants and animals. In the higher 

 plants food-building takes place in the green leaf cells provided 

 with chloroplasts ; circulation of foods, water, and salts occurs 

 in long chains of cells (ducts and sieve tubes); while assimila- 

 tion goes on in all living cells of the plant body. In the light 

 of these facts the organism is seen to be a great and highly dif- 

 ferentiated colony of cells, which, like the members of a civi- 

 lized community or of a factory, are organized to maintain the 

 welfare of the community as well as that of each individual 

 member. In the studies which follow, the student will be in- 

 terested in gaining at first hand a knowledge of the cell 

 structure and cell activities of plants upon which the above 

 generalizations of the modern cell theory are based. 



