CHAPTER VII. 



THE GENERAL SHAPES OF PLANTS. 



217. AMONG protophytes those exemplified by Pleuro- 

 coccus vulgaris are by general consent considered the simplest. 

 As shown in Fig. 1, they are globular cells presenting no 

 obvious differentiation save that between inner and outer 

 parts. Their uniformity of figure coexists with a mode of 

 life involving the uniform exposure of all their sides to 

 incident forces. For though each individual may have its 

 external parts differently related to environing agencies, yet 

 the new individuals produced by spontaneous fission, whether 

 they part company or whether they form clusters and are 

 made polyhedral by mutual pressure, have no means of main- 

 taining parallel relations of position among their parts. 

 On the contrary, the indefiniteness of the attitudes into 

 which successive generations fall, must prevent the rise of 

 any unlikeness between one portion of the surface and 

 another. Spherical symmetry continues because, on the 

 average of cases, incident forces are equal in all directions. 



Other orders of Protophyta have much more special 

 forms, along with much more special attitudes: their ho- 

 mologous parts maintaining, from generation to generation, 

 unlike relations to incident forces. The Desmidiacece and 

 Diatomacece, of which Figs. 2 and 3 show examples, severally 

 include genera characterized by triple bilateral symmetry. 

 A Navicula is divisible into corresponding halves by a trans- 

 134 



