CHAPTER VI 
PLANT TISSUES 
A tissue is an aggregation of cells of common source, structure 
and function in intimate union. 
THE TISSUES OF SPERMATOPHYTES AND PTERIDOPHYTES 
The tissues of seed plants and pteridophytes are all derived from 
a fertilized egg (oospore) which has undergone repeated divisions. 
At first either an apical cell arises or a mass of cells is formed which 
are essentially alike, but gradually we find that a division of labor has 
become operative setting aside many different groups of cells, each 
group of which has its particular role to perform in the economy of 
the whole. Each group of cells similar in source, structure and 
function is called a tissue. The tissues found in higher plants range 
from those whose component cells are more or less rounded, in a 
rapid state of division, and whose thin cellulose cell walls enclose 
a mass of protoplasm devoid of vacuoles or with exceeding small 
ones to those whose cells through various physical and chemical 
factors become compressed, elongated, and highly modified in respect 
to their contents and walls. 
As was shown by Hanstein , 1 the embryo of Angiosperms, while 
still constituted of only a few cells in the process of division, becomes 
differentiated into three layers of cells which differ in their arrange- 
ment and direction of division; these were called by him, Dermatogen , 
Periblem and Plerome. In roots a fourth layer of cells is sometimes 
evident at the apex. This was termed by Janczewski 2 the Calyptro- 
gen layer. These primary layers or groups of cells are called primary 
meristems or generative tissues. They are composed of more or less 
1 Hanstein, “Die Scheitelzellgruppe im Vegetationspunkt der Phanerogamen,” 
Bonn, 1868. 
2 Am. Sci. Nat. 5 s6rie, tom. xx. 
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