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, i. 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. 



Now it is 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 



