LEAVES 



577 



Hairs have been supposed to protect leaves against the injurious' 

 effects of heat, light, and cold. In particular, hairy coats on the upper 

 surface have been thought to screen off injurious light and heat rays, it 

 having been shown that hair-clad leaves become heated more slowly than 

 do smooth leaves. The hairs of aquatics 

 (as in Salvinid) are of undoubted service 

 in preventing leaf wetting, thereby facili- 

 tating unimpeded gas exchange. Stiff 

 hairs, as in mullein, and spiny hairs, as 

 in the thistles, probably afford some pro- 

 tection against grazing animals ; in pas- 

 tures, thistles frequently are untouched, 

 while other plants are eaten greedily. 



It must be admitted that the known uses of 

 leaf hairs are small in comparison with their 

 abundant development. While the discovery of 

 advantages now unknown is possible, it is much 

 more likely that most such hairs are of little or 

 no advantage. The idea should be abandoned 

 that plants have the power to discard organs that 

 are not of use. 



Stinging hairs. Stinging hairs are found 

 in various members of the nettle and spurge 

 families, and consist commonly of a large elon- 

 gated cell inserted in a cup-shaped emergence 

 (fig. 824). The cell walls are thick and brittle, 

 being silicified or calcified, and the enlarged end 

 is turned slightly to one side (figs. 825, 826). 

 When struck sharply, the hair ruptures obliquely 

 just below the head, leaving a sharp point that 

 suggests a hypodermic needle. The cell con- 

 tents, which are in a state of high turgor, rush 

 out, injecting an albuminoid poison into the 

 wound, if one is made by the broken hair (fig. 

 827). Nowhere in plants is there an organ more 

 clearly fitted for a definite function than are 

 stinging hairs, yet there is no evidence that they 



FIGS. 824-82 7. Stinging hairs : 

 824, a stinging hair from the wood- 

 nettle (Laportea, canadensis), a 

 unicellular structure seated on a 

 slight leaf emergence (e) ; note the 

 bulb-like base (6) containing the 

 prominent nucleus (); note also 

 the curved tip (t); considerably 

 magnified; 825, 826, the tips of 

 similar hairs, showing the thin 

 neck where oblique breakage oc- 

 curs; highly magnified; 827, the 

 tip of a broken stinging hair of 

 Urtica dioica, showing the poison- 

 ous contents flowing out; highly 

 magnified. 827 from HABER- 



LANDT. 



are of any special advantage to the plants pos- 

 sessing them. Nothing is known concerning the factors underlying their develop- 

 ment, since they neither vary appreciably nor grade obviously into other sorts 

 of hairs. 



The reduction of transpiring surface. The most fundamental dis- 

 tinction between xerophytic and mesophytic leaves is in the proportion 



