36 



THE TISSUES OF PLANTS 



lusca. At first these parts consist of cells more or less 

 closely connected together, either by a general enveloping 

 membrane, or by an intercellular substance which holds 

 them together by its adhesive properties. 



Fig. 4. 



Fig. 4 represents a portion of one of the animal layers 

 included between the calcareous laminse of a bivalve shell ; 

 in which are <shown at , nuclei forming in the midst of a 

 plastic fluid prepared and elaborated by the cells of a pre- 

 vious generation; b, the same advanced to the condition of 

 incipient cells; c, the cells more developed but still sur- 

 rounded by the fluid ; d, the cells in close contact with 

 each other, and rendered polygonal by mutual pressure. 



These last cells in the enamel of the teeth attract phos- 

 phate of lime into their cavities, whilst those which form 

 the shelly covering of the mollusca become filled with cal- 

 careous matter. The walls of the cells now disappear and 

 there is a coalescence of their cavities, so that the solid mass 

 appears altogether homogeneous, retaining not a single 

 trace of its cellular origin. 



In some cases, however, the cellular character of the 

 tissue is maintained throughout the life of the animal. 

 Thus, what is commonly called fat, consists of a mass of 



