30 



THE ECOLOGY OF STEMS 



The attack of a plant by a parasite usually results in an increased 

 development of conducting tissue. This is well seen in insect galls 

 and root tubercles as well as in stems, leaves and roots that are 

 attacked by fungi or other plants. Such an effect might be expected 

 since a parasitic attack increases activity and thus increases the use 

 of water and other materials and the result of this should be com- 

 parable to the result of a lack of these materials in the environment. 



Fig. 9. — Portion of cross-section of water lily petiole showing sclereids. 

 parenchyma cells; S, sclereid; A, air space. 



P, 



19. Mechanical Tissues.— Mechanical or stiffening and strength- 

 ening elements in plants are mainly of three general types. These 

 are fibers, or elongated sclerenchyma cells; sclereids, or sclerenchyma 

 cells that are not appreciably elongated; and collenchyma, or elon- 

 gated cells whose walls are not uniformly thickened but are thickened 

 primarily at the angles. 



