CLIMBING AND WATER PLANTS 123 



mum of distortion in vessels and cells which 

 lie in the central region of the stem. 



As far as aquatic plants are concerned, it 

 is only for those which inhabit torrents that 

 the mechanical tissue possesses much real 

 significance. In the ordinary vegetation of 

 ponds and sluggish rivers, a large number 

 of the stems of the submerged vegetation 

 are provided, it is true, with strands of 

 mechanical tissue, but they often appear to 

 be scattered rather at haphazard through the 

 substance of the stem as a whole. Indeed, 

 it almost seems as if these water plants had 

 rather free play in the differentiation of the 

 tissue in question. Ordinary aquatic condi- 

 tions are not constant or strenuous enough 

 to demand a high standard of mechanical 

 efficiency. Hence the less rigorously adapted 

 individuals are not eliminated, and the average 

 of the race in this respect is soon corre- 

 spondingly lowered. 



We may complete our survey of mechanical 

 tissues, and kindred matters, by briefly 

 considering the mode of construction of the 

 leaf from this point of view. 



The flattened shape of most foliar organs 

 evidently exposes them to risks of being 

 torn, and furthermore it is of prime import- 

 ance that the ordinary leaf should retain 

 an extended form, and not easily buckle; 

 otherwise the chlorophyll would cease to be 

 advantageously displayed to light. The 

 danger of buckling is partly met by the 



