LEAVES AND THEIR USES 231 



amount of water, and even a slight evaporation might cause 

 the wilting and death of the buds. Some plants (such as 

 many of the grasses and sedges) have stiff, sharp-pointed 

 hairs which, like thorns and spines, protect the plant from 

 destruction by hungry animals. 



Some hairs are organs in which special substances are 

 produced. Such hairs are glandular hairs; they are usually 

 swollen at or near the end, and in this swollen part of the 

 hair the substance it produces is stored. The glandular 

 hairs of the nettle have made a forcible impression upon 

 most of us ; it is a slightly poisonous liquid formed in them 

 that causes the unpleasant sting. The hairs of some tropical 

 nettles produce still more poisonous substances whose effects 

 are said to last for days. Nettles and plants with similar 

 glandular hairs are carefully avoided by grazing animals. 

 But the substances formed in the glandular hairs are by no 

 means always poisonous. The varnish-like substance which 

 covers the winter buds of the horse-chestnut is produced by 

 glandular hairs borne on the bud scales. The bracts and 

 ovaries of the pistillate flowers of the hop bear scale-like 

 glandular structures that produce the bitter substance 

 which makes hops valuable for medicinal purposes as well 

 as in the manufacture of beer. 



247. Shapes of Leaves. — One way in which species of 

 seed plants differ greatly from one another is in the shape 

 of their leaf -blades. These differences in shape are closely 

 connected with differences in the course of the veins in the 

 leaf. The leaves of most monocotyledons, like those of 

 com, are parallel-veined. In such a leaf, there is one vein 

 running close to, and parallel with, each edge of the leaf. 

 Parallel-veined leaves usually have entire margins — that is, 

 margins that are not indented. Some monocotyledons and 

 most dicotyledons have netted-veined leaves, many of whose 

 veins run out toward the edge and there end abruptly. It 

 is possible for the edge of the leaf to be indented or cut-in 



