CELLS AND TISSUES ii 



Suberization occurs in connection with the maturation of the 

 walls of cork cells, phellem. Priestley and Woffenden (34) have 

 considered the causal factors in cork formation and report that 

 there is first a blocking of the parenchymatous surface, which 

 usually is accomplished by the deposition of suberin or cutin in the 

 presence of air. This is followed by an accumulation of sap at the 

 blocked parenchymatous surface and the development of an active 

 phellogen in the parenchyma. 



In cutinization, the cutin may be deposited in all the walls 

 of the epidermal cell, or on the inner face of the outer epidermal 

 wall; but, most frequently, it is laid down on the outer surface of 

 this wall, forming a protective non-cellular membrane known as 

 the cuticle, which forms an impervious layer except where there are 

 stomata. The cuticle is variable in thickness, being well developed 

 on the epidermis of ripe fruits and the vegetative organs of many 

 xerophytic plants. 



Other chemical changes may occur in the epidermal walls of 

 some seed coats. In Linum, they become mucilaginous, as a 

 result of modifications of the pectic substances and cellulose of the 

 wall; and their capacity to hold water is greatly increased. Under 

 certain circumstances, mineral substances, carbonates, oxalates, or 

 silicates are deposited in cell walls. Silicates occur in relatively 

 large quantities in the peripheral cells of the stems of the grasses, 

 and an extreme case of mineralization takes place in the develop- 

 ment of cystoliths in the epidermal cells of Cannabis. 



TISSUES 



Strictly defined, a tissue is a group of cells of common origin 

 having essentially the same structure and performing the same 

 functions. Usage has broadened the scope of the term, however, 

 so that now it is necessary to refer to simple tissues, which conform 

 to this definition; and complex tissues, aggregations of simple 

 tissues, that have come to be regarded as having structural or func- 

 tional unity. This extension of the term to include complex tis- 

 sues such as xylem and phloem is convenient in descriptive 

 anatomy, but it should be noted that they refer to vascular com- 

 plexes consisting, in some instances, of four or more distinct cell 

 types rather than a single one. The tissue concept is further com- 

 plicated by duplication and lack of uniformity in the usage of terms, 

 the nomenclature used depending upon whether the point of view is 



