18 BOTANY. 



18.— Every free mass of protoplasm tends to assume a 

 spherical form. The free cells of the unicellular water plants 

 are generally more or less rounded, as are also the floating 

 spores of most aquatic Thallophytes. In plants composed of 

 masses of cells their mutual pressure gives them an angular 

 outline. Where the pressure is slight the cells depart but 

 iittle from the spherical shape, but as it becomes greater 

 they assume more and more the form of bodies bounded by 

 planes. If the diameters of the individual cells are equal 

 and the development of the ma?s of cells has been uniform 

 in every direction, we may have regular cubes, or twelve-sided 

 bodies, i.e., dodecahedra. It is rarely the case, however, 

 that the cells have a perfectly regular form. Even when 

 their diameters are approximately equal, they are generally 

 so much distorted that they are best described as irregular 

 polyhedra. 



19 — It much more frequently happens that cells gi'ow 

 more in some directions than in others, and thus give rise 

 to elongated and many irregular forms. In many of the 

 Thallophytes the long filaments composing the plants 

 are made up of elongated cylindrical cells placed end to 

 end ; while in others the cells are repeatedly and irregularly 

 branched. 



In higher plants many elongated cells occur, but here, 

 by pressure, they generally become prismatic in cross-section. 



(a) Many forms of cells have been enumerated, but tliey may alj be 

 arranged under the two principal kinds indicated above, viz., the 

 short, and the elongated. As will be more fully sliown hereafter, the 

 various kinds of short cells constitute what is called Parenchyma; 

 lience the cells themselves are termed Parenchymatous cells, or Paren- 

 chyma cells. Similarly, certain kinds of the elongated cells constitute 

 Prosenchyma, and hence such are termed Prosenchymatous cells, or 

 Prosenchyma cells. While it is impossible to draw an exact line be- 

 tween parenchymatous and prosenchymatous forms, yet the terms are 

 valuable, and are in constant use to indicate the general form. 



(b) Duchartre* has made an exceUent classification of the prin- 



* In his Elements de Botanique,'- second edition, a large and 

 valuable work, which the student may profitably consult. 



