CHAPTER VI 
PLANT TISSUES 
A TissuE is an aggregation of cells of common source, 
structure and function in intimate union. 
Tue Tissues oF SPERMATOPHYTES AND PTERIDOPHYTES 
The tissues of seed plants and pteridophytes are all derived 
from a fertilized egg (zygote) which has undergone repeated 
divisions. At first either an apical cell arises (pteridophytes) 
or a mass of cells is formed (spermatophytes) at the tips of grow- 
ing regions which are essentially alike and constitute the pri- 
mordial meristem; 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 com- 
pressed, elongated, and highly modified in respect to their 
contents and walls. 
CLAssIFICATION OF ‘TissuEs.—Tissues may be classified 
according to method of development, form, structure, function, 
and whether simple or complex. 
According to their method of development, tissues are either 
primary or secondary. Primary ‘Tissues are those which 
develop at the growing points of different organs by cell division. 
They arise from apical and intercalary meristems. They 
include parenchyma, epidermis, endodermis, pericycle, primary phloem, 
primary xylem, and cambium. 
111 
