158 Practical Plant Biology. 



tissues. The cells which are not in direct contact with these 

 passages or with the outer air, must obtain their oxygen supply in 

 solution from adjacent cells. The sluggish life of the plant which 

 is practically without movement, and does so very little external 

 work, explains how it is that gaseous and liquid diffusion through 

 these channels is sufficient to keep up the requisite supply. 



Life on the land necessitates mechanical properties different 

 from those which are suitable to aquatic organisms. These pro- 

 perties are chiefly associated with the supporting or skeletal 

 tissues. In water, buoyancy associated with a certain amount of 

 tenacity is sufficient to attain the exposure of the photosynthetic 

 organs to light, and to secure them against injury. Sub-aerial 

 plants must support their leaves on rigid structures to display them 

 to the light, but they must yield in wind and recover their position 

 when favourable conditions return. The requisite rigidity is ob- 

 tained by a suitable disposition of sclerenchyma and the elastic 

 recovery is usually provided for by a combination of parenchyma 

 with sclerenchyma or by means of parenchyma alone. Thus the 

 bases of the leaf-stalks which must always stand erect are con- 

 structed out of a thick-walled tube of sclerenchyma forming 

 their superficial tissue. The cells of this sclerenchyma have thick 

 hard walls and they are not easily stretched or compressed so that 

 the lower part of the stalk is very rigid. To displace it at all one 

 side must elongate and the other contract. If, however, any dis- 

 tortion is produced, evidently this tube must be flattened and its 

 volume correspondingly reduced. Such a reduction is not only 

 resisted by the walls of the tube but is opposed also by the cells 

 of the fundamental tissue within it. Any pressure on them tends 

 to stretch their walls, for their vacuoles are practically incom- 

 pressible, and hence a tension on every cell-wall throughout the 

 tissue acts to restore the original position of the leaf-stalk. In 

 the upper parts of the rachis where there is less sclerenchyma and 

 more thin walled tissue, the rigidity of the organ diminishes 

 but its flexibility increases. Here the flexure of the rachis is 

 chiefly resisted by the walls of the parenchyma. The bending 

 produces a pressure on the cells which distorts them more and 

 more from the spherical form. Owing to the incompressibility of 

 the vacuole their volume must remain the same, therefore their 

 wall is stretched. When the bending force is removed the 

 pressure disappears and the elasticity of the cellulose walls restores 

 the cells to their more spherical original shape. In this region 

 the sclerenchyma or mechanical tissue as it is called is not 

 continuous round the rachis and consequently only acts like 



