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 
