THE PLANT CELL 77 



PLANT TISSUES 



Immediately upon fertilization of the egg-cell 

 contained in the ovule, multiplication of cells by the 

 process of mitosis occurs, and an embryo is formed. 

 In the earliest stages of embryo formation the cells 

 produced by indirect division are alike in structure, 

 but a differentiation of cells soon occurs in the develop- 

 ment of the embryo. This differentiation ultimately 

 results in the formation of tissues, or groups of cells 

 having specific characters. As previously noted, each 

 tissue has a certain function and is particularly fitted 

 to perform this function by certain peculiarities in 

 the structure of its cells. In the young embryo 

 practically all the cells are capable of division; but 

 in the mature plant this property is present in com- 

 paratively few cells and these form the meristematic 

 tissues located at the growing points of stems and 

 between the bark and wood. The young embryo 

 may be divided into three zones of undifferentiated 

 cells, an outer or dermatogen region from which the 

 first covering tissues of the plant are derived; a middle 

 or periblem region from which the covering tissues 

 of the mature plant are derived; and a central or 

 plerom region from which all other plant tissues are 

 derived. Each of these zones (Plate 30), contains 

 meristematic cells which soon produce primary tissues 

 showing differences in structure and contributing to 

 the rapid development of the young plant. 



Primary Root Tissues. — Differentiation of the 

 fundamental tissue of the dermatogen, periblem and 

 plerom zones results from changes in the character 

 of the cell walls and from the activity of meristematic 



