PRELIMINARY DESCRIPTION OF THE CELL 27 



of new types of protoplasmic structure or cellular organization, but in 

 the development of constantly greater specialization and division of 

 labor between larger and larger groups of cells." One obvious reason for 

 the fundamental similarity of the cells of widely different tissues is found 

 in the fact that all of them are derived by progressive modification from 

 relatively undifferentiated "embryonic" or "meristematic" cells during 

 the course of the ontogeny. In a young vascular plant, for example, 

 definite regions (meristems) consisting of such cells are present in the 

 root tip and stem tip, and, at a later stage of development in many 

 cases, in the cambium also. As a general rule these meristematic cells 

 are without large vacuoles or other conspicuous products of differentiation, 

 and are separated by no intercellular spaces. They undergo successive 

 divisions very rapidly (hence the use of root tips for the study of mitosis) ; 

 and while some of the products of division become greatly modified in 

 structure in connection with their specialization in function, others 

 retain their embryonic or meristematic character and continue to produce 

 new cells from which new tissues are built up throughout the life of the 

 plant. 



In the bryophytes and pteridophytes the meristematic activity of the 

 apex (root tip or stem tip, or apical region of thallus) usually centers in 

 a single "apical cell" of definite shape, which cuts 

 off segments (daughter cells) from its various faces 

 with great geometrical regularity. In the Mar- 

 chantiales and Anthoceros the apical cell is cuneate 

 (wedge-shaped) and forms segments from four of 

 its faces; in the anacrogynous Jungermanniales 

 it is sometimes cuneate but more often dolabrate 

 (ax-shaped) and produces segments from its two 

 lateral faces ; and in the acrogynous Jungermanniales 

 and mosses it has the form of a triangular pyramid, 

 cutting off segments from its three lateral faces. 

 This last type is found also in the pteridophytes : 

 in the stem tip it produces segments from its three section of the root tip of 

 lateral faces, whereas in the root tip, in addition Osmunda, showing the 



triangular pyramidal 



to these three series of segments, it cuts off from its apical cell, x 144. 

 distally directed face a fourth series, which becomes 



the tissue of the root cap (Fig. 3). In the higher vascular plants there 

 is no single cell characteristically different from the others of the apical 

 meristematic group. 



Most of the visible characters which ordinarily serve to distinguish 

 the various kinds of differentiated cells of the vascular plant are found 

 in the cell wall rather than in the protoplast itself; strictly speaking, 

 such characters are histological rather than cytological. Thus we have, 

 besides meristematic and little modified parenchymatous cells (Fig. 2, 



