THE CELL 19 



minuteness in the details of its mechanism which transcend our 

 comprehension and baffle the human imagination, to the same 

 extent as do the immensities of the stellar universe." The 

 processes of cell growth and division will be much better under- 

 stood later in our course and will be taken up again in Chapter X. 



What is a Plant ? In speaking of plants we usually refer to 

 the higher plants which are very complex in anatomical structure 

 and in physiological activity. But, if we think of the plant 

 kingdom as a whole, we find a wonderful range from the very 

 simple microscopic, unicellular plant to the very complex plants 

 referred to above. The simplicity applies to the structure rather 

 than to the functions, for we find the unicellular plants perform 

 very complex physiologic functions. If we make a study of a 

 series of plants from the lowest to the highest (see Chapters 

 XXIII and XXX) we find a gradual modification from plants 

 composed of one or a few cells of the same character to plants 

 composed of great numbers of cells which are very different in 

 structure and which are grouped in a very definite manner, and 

 finally by the formation of very definite plant organs, such as 

 leaves, stems and roots. This gradual modification and grouping 

 of cells is accompanied by a corresponding change in physio- 

 logical functions. In the lower plants each cell may perform 

 any or all of the functions of the plant, but in advancing from 

 the lower to the higher groups we find a distribution of func- 

 tions among the modified cells and finally very evident divisions 

 of labor among the cells and organs of the plants. 



We will begin our work by studying the higher plants, those 

 with which we are most familiar. These higher plants are very 

 complex in both structure and function. In general, they con- 

 sist of three primary parts : stems, roots and leaves. The various 



