HUTCHINSON'S POPULAR BOTANY 



greatest. This is well illustrated in the leathery leaves of the Holly 

 (Ilex) and the Indiarubber-plant (Fiats elasiica). 



Leaves which assume a vertical position are specially exposed to the 

 violence of the wind. The currents of air usually take a course which is 

 parallel to the earth and therefore strike against such leaves at right angles, 

 so that special adaptations are needed to enable the latter to retain their 

 upright position. In many of the Grasses the 

 Common Reed (Phragmites communis) may serve as 

 an example the leaf-blades turn on the haulms 

 (which is the stalk of a grass of any kind) 

 like weathercocks. In the Reedmace (Ty-pha lati- 

 folia, fig. 349) the leaf is spirally twisted, so that a 

 whole surface is never presented to the wind an 

 arrangement the advantage of which is sufficiently 

 obvious In other plants, protection from the wind 

 is secured by the leaf being hollow. It is well 

 known that a tube resists flexion more effectually 

 than a solid body; and tubular or fistular leaves will 

 maintain their erect position even in the roughest 

 weather. Examples of the fistular leaf are pre- 

 sented by the Common Onion (Allium cepa) and 

 other bulbous plants. In the Purple Crocus (C. 

 oflicinalix) the edges of the leaf roll over towards 

 the white central stripe so as to form a sort of 

 double tube ; and thus this little harbinger of spring 

 is able " to take the winds of March with beauty " 

 (fig. 351). 



When speaking of buds, we showed that the 

 chief purpose of the woolly growth which often 

 covers them is to protect the young leaves from the 

 cold winds and nipping frosts of winter. It must not 

 be imagined, however, that this is also the chief pur- 

 pose of the wool and hairs which cover more or less 

 thickly the surfaces of many adult leaves. Heat, 

 rather than cold, is the danger to which the mature 

 leaf is exposed, and the purpose of its covering 

 hairs is not so much to promote warmth as to pre- 

 vent excessive exhalation. Just as the succulent stems of the Cactuses and 

 many tropical Euphorbias are provided with a leathery membrane to retard 

 evaporation, so, and for the same reason, a great number of leaves are 

 provided with hair-like structures, which, by shielding the epidermis from 

 the direct rays of the sun, reduce transpiration and save the leaves from 

 untimely desiccation. 



END OF VOL. I 



FIG. 351. CROCUS. 



A two-barrelled fistular leaf. 



