300 



HUTCHINSON'S POPULAR BOTANY 



terminal leaflet assumes the perpendicular position which has been already 

 shown to be characteristic of " sleeping " leaves. 



The trembling movement of the leaves of the Aspen (Populus tremula) 

 has supplied many figurative allusions to prose-writers and poets, and the 

 phenomenon deserves a passing notice. The quivering is due to the 

 elasticity of the long flattened foot-stalks ; and Mr. Colbourn, of Hobart, 

 suggests that the rapid movement in the air enables the leaf to throw off 

 the excess of moisture which collects on it in the damp situations of the 

 tree. Some force is given to this view if we look at the Aspen or the Black 

 Poplar immediately after rain, when we shall find great numbers of the 

 leaves held together bv moisture. Kerner, however, regards the motion as 

 an arrangement for protecting the flat broad 

 leaves against crushing ; but many other broad 

 flat leaves are without this provision. He 

 further remarks that the elasticity is due to the 

 development of bast-strands in the leaf-stalks. 

 We have now considered a few of the dan- 

 gers to which the green leaves of plants are 

 exposed, but the subject would be very im- 

 perfectly treated were no mention made of a 

 danger of another kind. This form of danger 

 belongs to the animate rather than the inani- 

 mate world to " wild beasts and beasts of the 

 field and creeping things'' rather than to heat 

 and cold and other such phenomena and forces. 

 Innumerable animals feed upon the green 

 tissues of plants, and find in a vegetarian diet 

 their only sustenance; indeed, if Nature had 

 not provided special contrivances to keep off 

 these devourers, it is next to certain that whole 

 families of plants would long since have van- 

 ished from the face of the earth. A few of 

 these contrivances have been incidentally re- 

 ferred to in former chapters. When speaking of the sap of plants, we 

 showed that the milky juice of the Common Lettuce (Lactuca sativa) pro- 

 tected the plant from the depredations of ants and other leaf-eating insects; 

 and on a later occasion we saw that the thorns or spines in such plants as 

 Blackthorn (/Yunua spinosa). Spiny Restharrow (Ononis spinosa), 

 bpurges (Euphorbia), etc., render acceptable service by keeping off brows- 

 ing cattle and herbivorous wild animals. But the subject was only lio-htlv 

 ouched, and-from the nature of the connection in which it was intro- 

 uced-many of these protective contrivances were not alluded to at all. 

 ^or example no mention was made of pricttes. Prickles are another 

 thorn. They are not, like spines, branches which have degenerated, 



FIG. 366. A PRICKLY PEAR 

 CACTUS (Opuntia multiflora), 



Protected by fine barbed bristles. 



