THE STEM. 



101 



shapes, such as hairs, scales, and the like (figs. 113, 114; 

 see also figs. 361-365). 



129. 2. The cortex. — In some plants the cortex under- 

 goes an enormous development, forming in some tubers the 

 greater part of the massive stem. 

 In other plants the cortex under- 

 goes such reduction that it con- 

 sists only of two or three layers of 

 cells. It very commonly enters 

 with the epidermis into the for- 



FlG. 113 



Fig. 113. — Forms of hairs from Plectranthus. a, simple pointed hair; /•, stalked 

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

 cells. Highly magnified. — After De Bary. 



Fig. 114. — T-shaped hair of the wall-rlower [Cheiranthus). e, epidermis. Highly 

 magnified. — After De Bary. 



mat ion of outgrowths, which are then known as emergences. 

 These emergences may take the form of rounded elevations, 

 producing a warty stem, or they may be sharp pointed 

 and either straight or curved, forming prickles (figs. 115, 

 116); or the emergence may be produced along a con- 

 tinuous line, giving rise to wings upon the stem ; or the 

 stem may be more or less covered with large pointed or 

 angular elevations, called tubercles, as in some cactuses 

 (fig. no). Very frequently the intercellular spaces of the 

 cortex are greatly enlarged, forming air passages of con- 

 siderable size. These passages may arise by mere separation 

 of the cells of the cortex, or by the destruction of those in 

 certain regions, or by a combination of these causes 

 (fig. 117). In other cases the cortical cells, instead of 



