86 OUTLINES OF PLANT LIFE. 



2. The cortex consists of several layers of cells, usually 

 thin-walled and not in close contact, and hence abundantly 

 provided with intercellular spaces. These cells usually con- 

 tain many chloroplasts, to which the green color common to 

 young stems is due. 



3. The stele forms the central region. Its most striking 

 parts are several or many clusters of smaller cells, the cut ends 

 of the vascular strands. Occupying the space between the 

 vascular strands is the pith (figs. 72, 73). 



108. The cortex. — In certain plants the cortex undergoes 

 an enormous development, forming in some tubers the 

 greater part of the massive stem; 

 in others it is so reduced that it 

 consists only of two or three layers 

 of cells. With the epidermis it 

 very commonly enters into the for- 

 mation of outgrowths, such as 



Fig. 74. Fig. 75. 



Fig. 74. — Forms of hairs from Piectranthus, a, simple pointed liair; h, stall^ed 



glandular hair ; c, sessile glandular hair with secretion covering the two glandular 



cells. Highly magnified. — After De Bary. 

 Fig. 75. — T-shaped hair of the wall-flower {Cheiranfhui^. e, epidermis. Highly 



magnified.— After De Bary. 



warts, prickles, wings, etc. Very frequently the intercellular 

 spaces of the cortex are greatly enlarged, forming air passages 

 of considerable size (fig. 76). In other cases the cortical 

 cells, instead of remaining thin-walled, may become greatly 

 thickened in certain regions, or even throughout the cortex. 

 These mechanical cells are likely to be aggregated in clusters 

 or strands, and serve an important purpose in strengthening 

 the stem. 



