MECHANICAL CONSTRUCTION OF PLANT-BODY 143 



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, 102. 

 Collcnchyma from stem of the Potato, seen in transverse section, ( x 300.) 



between the action of each. The 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) CoUenchyma, 

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

 chyma, which is characteristic of more mature parts. CoUenchyma 

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

 physiologically active, while chlorophyll corpuscles 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. 102. It is thus a tissue which, 



