66 PART II. ANATOMY AND HISTOLOGY. [ 21 



tissues of a histologically differentiated body present characteristic 

 peculiarities of form, size, and relative arrangement, the most 

 striking distinctive peculiarities are exhibited, not, as in animals, 

 by the protoplasm of the cells, but by the cell-walls in respect of 

 their thickness, their chemical composition and physical properties, 

 and their markings. 



Inasmuch as the cellular plants are the more numerous, and 

 present greater variety of structure, the following account deals 

 almost exclusively with them. And since the cell is the structural 

 unit of these plants, it will be advantageous to study the cell as 

 such first, and then to proceed to the study of the tissues. 



CHAPTER I 



THE CELL 



21. The Structure and Form of the Cell. In a fully 

 developed living cell the following three principal constituents 

 may be distinguished (Fig. 36 B, C and Z>) : 



(1) A closed membrane, ihe^ell-wall (7i), consisting generally of 

 a substance termed cellulose. 



(2) A layer of semi-fluid substance, the jtrotojolasm (p\ lyingjn 

 close contact at all points with the internal surface of the cell-wall ; 

 the protoplasm gives the chemical reactions of proteid. In it lies 

 a nucleus ,(fc), in which one or more smaller bodies, nucleoU (kk) 

 may generally be distinguished. 



(3) Cavities, one or more, in the protoplasm, termed vacuolcj (s), 

 which are filled with a watery liquid, the cell-sap. 



The structure of a ccenocyte is essentially the same as that just 

 described, except that several (sometimes very many) nuclei are 

 present. 



The young cell presents a somewhat different appearance (Fig. 

 36 A). At this stage the protoplasm occupies the whole cell- 

 cavity. But, in the subsequent development of the cell, the in- 

 crease in bulk of the protoplasm does not keep pace with the 

 superficial growth of the cell-wall. Hence, since thejjrptopjasm 

 must remain in contact with the cell-wall at all pjjinJts, the result 

 is that cavities, the vacuoles, are formed which become filled with 

 cell-sap (Fig. 36 5). The vacuoles, small at first, increase with 

 the growth of the cell, and may fuse together to a greater or less 



