PROTECTIVE TISSUES 127 



so that the different tissues may be enlarged as the growth of the 

 plant demands. By means of a meristem at their tips, roots 

 and stems elongate, and by means of the cambium they in- 

 crease in circumference. However, the meristems at the growing 

 apices are the first sources of all other tissues, even of the cam- 

 bium, and for this reason are known as the primary meristems. 

 (Fig. 111.) 



Meristematic cells are characterized by having thin cellulose 

 walls, large nuclei, and dense cytoplasm features which enable 

 the cell to grow and divide rapidly. Closely related to the meri- 

 stematic cells are the parenchyma cells, which also in most cases 

 have thin cellulose walls but are less active in dividing. Paren- 

 chyma cells occur scattered throughout the various plant tissues 

 and constitute the food-making tissues of leaves and stems, and 

 most of the pith of plants. 



Protective Tissues. For protection against destructive agen- 

 cies plants have their outer cells modified into protective tissues, 





3 



B 



FIG. 112. A, epidermis of a leaf showing epidermal cells (e) with their 

 outer cutinized walls (c). B, the flesh (j) and rind of a Jonathan Apple show- 

 ing the thick, cutinized, outer walls (c) of the epidermal cells (e). Much 

 enlarged. 



such as epidermis, corky rind, and bark, which lessen evaporation 

 and prevent the entrance of destructive organisms. The most 

 common protective tissue is the epidermis which consists of one 

 or more layers of cells forming a jacket about the plant. The 

 outer walls of the exterior layer of epidermal cells are usually 

 thickened and contain a waxy substance called cutin which makes 

 them waterproof. (Fig. 112.) Most plant organs are at first pro- 

 tected by an epidermis, but in the older portions of stems and roots 

 the epidermis is often replaced by cork tissue, which is usually 



