Z7. THE STRUCTURE OF ECONOMIC PLANTS 



An endodermis of this type, in either the secondary or tertiary 

 stage, is frequently found in roots, underground stems, and the 

 stems of hydrophytic plants. In the aerial portions of the axis of 

 herbaceous plants, the endodermis generally remains in the primary 

 condition without developing Casparian strips; and, if it differs 

 from the adjacent tissues at all, it is in the greater starch content of 

 its cells, in their shape or size, and in the uninterrupted continuity 

 of the layer. 



MECHANICAL TISSUES 



The tissues which give strength, and those that provide tough- 

 ness combined with elasticity and flexibility, constitute the me- 

 chanical elements of the plant body. These include collenchyma, 

 non-sclerotic fibers, sclerenchyma, and the lignified elements of the 

 vascular system. The sclerenchyma may consist of elongated 

 sclerotic fibers or approximately isodiametric stone cells. 



CoLLENCHYMA. — This simple tissue has many characteristics of 

 parenchyma and may be regarded as a derived form of it. The cells 

 retain their protoplasts at maturity, are capable of further division, 

 and differ from parenchyma chiefly in the manner in which the 

 walls are thickened. The cells are axially elongated, rounded, or 

 more frequently angular, and four- to six-sided in transection, with 

 end walls that are straight, oblique, or tapering. The elastic 

 walls have simple pits, are readily permeable to water, and are 

 composed chiefly of cellulose although there may be some lignifica- 

 tion in old cells. The wall thickenings are laid down in longi- 

 tudinal strips which are commonly located at the angles of the 

 cells, but the tangential walls, and less frequently the radial walls, 

 may be thickened. (Fig. X4i.) 



In the stems and petioles of herbaceous plants, collenchyma 

 forms discrete zones or continuous bands of mechanical tissue 

 immediately beneath or near the epidermis. It also occurs in the 

 blade of the leaf, forming strands that parallel the midrib above 

 and sometimes below it. Differentiation of collenchyma begins 

 while the organs are still developing, and it is ordinarily the first 

 type of mechanical tissue to mature in the ontogeny of the stem 

 axis. This may be correlated with the fact that collenchymatous 

 cells retain their capacity to divide and grow which is an essential 

 characteristic for mechanical tissues located in regions that are 

 undergoing elongation. 



