698 ECOLOGY 



capable of growth elongation, and the walls are much more refrangible. 

 In addition to the permanent collenchyma, which is characteristic of the 

 cortical region of stems and petioles, temporary collenchymatic thicken- 

 ing often occurs in bast primordia. 



Sclerenchyma. — Representative sclerenchyma cells or sclereids are 

 mechanical cells that cannot be classified as collenchyma or as bast, 

 though bastlike fibers in the cortex often are called sclerenchyma. The 

 most characteristic sclereids are the hard, stiff, and relatively isodia- 

 metric stone cells with brownish lignified walls, which are found in the 

 secondary bark of trees and in the shell of the hickory nut, and which 

 arise through the sclerosis of ordinary parenchyma cells (fig. 1022). 

 Sclereids of this sort have stratified walls due to differential centripetal 

 growth, the walls being traversed by more or less branched canals, along 

 which the structural materials probably have passed. The cells die 

 soon after stratification ceases. 



The stellate mechanical cells of water lily leaves (fig. 805) and the T-shaped 

 prop sells of Osmanlhus (fig. 937) may be classed with sclereids. The rigidity of 

 stems is not due entirely to bast, collenchyma, and sclerenchyma, other tissues with 

 thickened walls, such as the wood and the cutinized epidermis, playing an important 

 part. In Equisetum and in various grasses (notably the cereals), considerable silica 

 is deposited in the walls, giving them considerable rigidity. 



The distribution of stereome in plants. ■ — Many sclereids occur as 

 idioblasts, being scattered irregularly through various tissues, as in the 

 bark of trees. More commonly the mechanical elements are grouped in 

 strands, the most usual condition being the association of mechanical 

 and conductive elements into a fibrovascular bundle. As a rule, com- 

 pact strands of bast fibers occur just outside the leptome (fig. 760), and 

 sometimes there is a mechanical cylinder entirely surrounding the vas- 

 cular bundle (fig. 1028), or even the whole vascular region (fig. 1027). 

 In monocotyls (especially in xerophylic species) there usually is a pro- 

 gressive decrease of bast strands inward, the outer bundles having 

 conspicuous mechanical strands outside the leptome, while often the 

 inner bundles are without them. In many xerophytic leaves and stems 

 there often are cortical strands of bastlike fibers, especially just beneath 

 the epidermis; similar strands are frequent also in the bark. Wood 

 fibers rarely are grouped in compact strands, but individual fibers with all 

 intergrading stages are scattered here and there among the conducting 

 elements. 



