52 THE ELASTICITY AND COHESION OF THE PLANT-BODY 



In the higher plants certain cells always develop thicker walls than 

 the rest, and obviously have mainly a mechanical strengthening function. 

 As the cell-wall thickens, diosmotic exchanges become increasingly difficult, 

 so that the protoplast ultimately perishes in the sclerenchymatous cells of 

 the cortex, pericycle, phloem, and wood when these elements are fully 

 formed. The plant in this way sacrifices certain of its living protoplasts 

 for the benefit of the remainder, and frequently living cells have their 

 powers of exchange diminished by a thickening of the cell-wall sufficient to 

 endow particular tissues with the necessary strength. Instances of such 

 are afforded by collenchyma cells, as well as by the common occurrence 

 of secondary thickening in the cortical cells of woody branches. It must 

 also be remembered that dead elements may subserve other functions, as 

 is instanced by the transport of water in dead wood-elements, and by the 

 importance of cork in the regulation of transpiration. 



The strengthening parts are arranged in all the higher plants in the 

 form of a skeletal framework, the meshes of which are filled by the softer 

 cells and tissues. The latter play an important part in joining together 

 the strengthening framework, and so increasing the rigidity of the whole. 

 Thin-walled turgid parenchyma, though easily torn by longitudinal ten- 

 sions, can withstand considerable pressure. The union with longitudinally 

 elongated bast and wood-fibres avoids the former danger, while the 

 varied characters of the strengthening cell-walls, as well as their 

 different arrangements, render possible various degrees of flexibility in 

 plants and their organs. In this respect it is of great importance that 

 the plant should be able to produce walls of different elasticity from 

 similar materials. 



In spite of individual differences, a general purposeful arrangement is 

 recognizable, and the plant endeavours to gain sufficient strength with 

 a minimum amount of strengthening materials. Thus frequently the 

 strengthening cells form a continuous or interrupted hollow cylinder, the 

 parts of which are joined together by the softer tissues or by special 

 cross-ties, which counteract lateral strains and torsions, and prevent rupture. 

 Similar arrangements to those adopted by plants are also employed by 

 engineers to economize their constructive material. For example, a hollow 

 cylinder, or a system of tied girders, gives a much more rigid structure, 

 and can bear a greater strain than a solid rod composed of the same 

 materials and having the same sectional area. The hollow stems of 

 grasses are, therefore, mechanically advantageous. Furthermore, in long 

 leaves and flattened stems in which bending is practically possible only in 

 one plane, the mechanical cells are mainly present just beneath the upper 

 and under surfaces, and this arrangement corresponds very closely to 

 a double series of tied girders. Where mainly longitudinal tensions 

 are involved, as in roots, and in the stems and petioles of certain 



