96 PLANT LIFE 
Exactly the same happens in the plant. 
When a sunflower, or a wheat stem, is bent 
by the wind the tissue on the outer side is 
stretched, that on the concave side is com- 
pressed, and it is easy to see that both con- 
ditions tend to straighten the stem again. 
Since the axis of the stem is neither pulled 
nor compressed, it is obvious that there 
would be no advantage in placing the mechan- 
ical tissue where the pith is; the further away 
from the axis, 2. e. the nearer to the circum- 
ference, the more effective it becomes. 
But the wind does not act only in the east- 
west plane, and plants are apt to be subjected 
to stresses from any and all sides. Thus the 
girder systems are more complex in their 
arrangement, and are so multiplied as to be 
ready and meet the stress from whatever 
quarter it comes. Moreover, they commonly 
receive additional rigidity by being tied 
together, in a tangential direction as well as 
transversely, by specially strong tissues, at the 
nodes. 
Nowitis evident that this form of mechanical 
tissue is not suited for all the conditions that 
may be experienced by stems. For example, 
the young parts are often elongating, and 
sclerenchyma is far too little extensible to 
admit of this growth. On examining such 
a growing region we find that although the 
sclerenchyma strands are recognisable there, 
they are not yet functional. In fact the cells 
which compose them are only beginning to 
