MECHANICAL CONSTRUCTION OF PLANT-BODY 175 



Rigidity as based on Specific Mechanical Tissues. 



Such methods serve to give the necessary mechanical strength to 

 young parts. But as the tissues grow older, their walls become thick- 

 ened. They are then less susceptible to turgor, as they are also more 

 resistant to growth. Moreover, in the older parts the mechanical 

 demand for support increases with the increasing burden of leaves 

 and branches. These demands are met by specific mechanical tissues, 

 fitted by their thickened walls to offer greater resistance. Though the 

 effect of turgor is characteristic of young plants, and that of the 

 specific mechanical tissues of the mature, there is no definite limit 



Fig. 109. 



Collenchyma from the stem of the Potato, seen in transverse section. 

 The walls only ara shown. ( x 31,0.) 



between the action of each. Dependence on the one merges gradually 

 with age into dependence on the other, and in the growing part both 

 sources of support may be effective at the same time. 



There are two types of specific mechanical tissue, (a) Collenchyma, 

 which is found in growing herbaceous stems and leaves ; and (b) Scleren- 

 chyma, which is characteristic of more mature parts. Collenchyma 

 consists of cells which retain their living protoplasts, and thus remain 

 physiologically active, while chloroplasts are frequent in its cells. 

 These cells usually have the form of 4-6-sided prisms, with transverse 

 or oblique ends, and are sometimes transversely partitioned. The 

 cell-walls are composed of cellulose, which is swollen in life with 

 water. They are thicker at the angles of the prisms than at their 

 flattened sides, where the thinner membrane allows of ready physio- 

 logical interchange. This gives the tissue, when seen in transverse 

 section, the appearance shown in Fig. 109. It is thus a tissue which, 



