CHAPTER II. 



CELLS IN THEIR MODIFICATIONS AND KINDS, AND THE 

 TISSUES THEY COMPOSE. 



190. "While ciyptogamous plants of the lower grade may 

 consist of single colls, or of a series or stratum of simple and 

 undifferentiated cells, pliiBnogaraous plants, although equallj' 

 simple and homogeneous at the initiation of each individual, 

 develop into a more complex organization, at an earl^' period 

 differentiate some of their cells into peculiar kinds, multiply the 

 kinds into tissues or fabric, and of these build np the organs 

 and parts which are familiar in ordinarj- vegetation. 



191. The inicroscropical studj- of the parts even of a single 

 herb or tree, and much more that of a variety of plants, reveals 

 numerous forms or kinds of cells, and also (as might be expected 

 from their common origin) brings to view series of gi'adations 

 between the kinds, sometimes even between those which are, 

 upon the whole, widely differentiated from each other. While, 

 therefore, a general classification of the cells of any ordinary 

 plant into kinds is easj-, any classification which shall satis- 

 factorilj- exhibit our present knowledge of the histological ele- 

 ments, and discriminate their varieties, is very difficult, if not 

 at this time practically impossible. At least, it must be said that 

 the most recent classifications are based upon considerations 

 of a character too recondite and special to be mastered at the 

 beginning by an ordinarj- student. 



192. The most general and obvious division of the histological 

 components of a stem, root, or leaf would be into, (1) funda- 

 mental or typical cells, and (2) transformed cells. The first are 

 those in which the normal cellular character persists without pro- 

 found, if an^-, alteration or disguise ; as in the pulp of leaves, the 

 pith of stems, and in a portion of the bark. The second are those 

 which assume or affect lengthened or fibrous forms and a longi- 

 tudinal development (at least in all axes, and commonly in leaves 

 and other expanded organs), and, combined into threads, fasci- 

 cles, bundles, or more massive structures, constitute the frame- 

 work, which imparts solidity and strength throughout. Some 



