LEAVES 



575 



of the rose (see figs. 1094-1096 and adjoining text). It is conceivable that the 

 influence of gall insects upon hair production is essentially comparable to that of 

 xerophytic factors, especially if the insects introduce osmotically active substances 

 into the plant (see p. 785). 



Figs. 821 822. — -Variation in Polygonum amphibium : 821, a branch from the 

 aquatic form with floating leaves borne on a, submersed stem; note the lax horizontal 

 stem, the smooth leaves, and the inconspicuous stipules (o); this represents the "typical" 

 P. amphibium; 822, a branch from the land form (often erroneously called P. Hart- 

 wrigMii) ; note the stout, erect emersed stem, the stiff hairy leaves, and the prominent 

 sheathing stipules (0). 



Whatever may be the influence of desiccation or of insect activity upon hair 

 production, neither these nor other external influences appear to modify hair form 

 to any great extent. In most species the shape and structure of the hairs seem 

 inherent, external factors determining only their presence or their absence. In 



