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, Derma- 

 togen, Periblem and Plerome. In roots a fourth layer of cells is 

 sometimes evident at the apex. This was termed by Janczewski 2 the 

 Calyptrogen layer. These primary layers or groups of cells are 

 called primary meristems or generative tissues. They are composed 



1 Hanstein, "Die Scheitelzellgruppe im Vegetationspunkt der Phanerogamen," 

 Bonn, 1868. 



2 Am. Sci. Nat. 5 serie, torn. xx. 



99 



