CHAPTER IX. 



THE PLAJ^T-BODY. 



§ I. GrENEKALIZED FOEMS. 



18-1. — The cells, tissues, and tissue systems described in 

 the preceding pages are variously arranged in the different 

 -groups of the vegetable kingdom to form the plant-body. 

 The simplest plants are single cells or undifferentiated 

 masses of cells ; in those next higher the cells are aggre- 

 gated into simple tissues, while still above these the tissues 

 are grouped into tissue systems. With this internal differ- 

 entiation there is a corresponding differentiation of the ex- 

 ternal plant-body. The lower plants are not only simpler as 

 to their internal structure, but they are so as to their exter- 

 nal form as well. The higher plants are as much more 

 ■complex than the lower ones as to their externalparts as 

 they are in regard to their tissues and tissue systems. 



165. — In the lowest groups of plants the simple plant- 

 body has no members ; the single-or few-celled alga has no 

 parts like root, stem, or leaf ; it is a unit as to its external 

 form. In the higher groups, on the contrary, the plant- 

 body is composed of several to many less or more distinct 

 members. In those plants in which they first appear, the 

 members are not clearly or certainly to be distinguished from 

 the general plant-body ; but in the higher groups they be- 

 come distinctly set off, and are eventually differentiated into 

 a multitude of structural and functional forms. 



166. — As will be seen in the future chapters, every plant, 

 in its earliest (embryonic) stages, is simple and memberless ; 

 and every member of any of the higher plants is at first indis- 

 tinguishable from the rest of the plant-body ; it is only in 



