88 



GROWTH AND WORK OF PLANTS 



leaf, so that the water is conserved. These leaves often closely 

 overlap or lie close against the stem as scales. The cassiope 

 (Cassiope tetragona}, which is found in sphagnum moors in some 

 of the Northern States, and is common from Labrador to Green- 

 land and Alaska, is an example. 



149. Needle-like leaves. These are found on many coni- 

 fers, especially the pines. The leaves are long, narrow, and 

 thick, and are called needle leaves. They have a thick, waxy 

 cuticle, an epidermis with thick walls. Beneath the epidermis 

 there are several layers of cells the walls of which are very thick 

 and hard, and inside is the mesophyll. This form and structure 

 of the pine leaves enables them to conserve water so that they 

 lose it very slowly; otherwise the leaves would lose so much water 

 that in winter the trees would be killed. The spruces have similar 

 leaves, but they are shorter and more flattened, while some other 

 evergreens have scale leaves, which with their structure enables 

 them to endure the drying effect of the cold winters. 



150. Modifications of leaves combining the normal func- 

 tions with other utilities. First, tendrils and tendrils on 



leaves, as in the pea; 

 also where the petiole 

 of the leaf functions as 

 a tendril, as in the vir- 

 gin's bower (Clematis}. 

 Second, the leaves of 

 insectivorous plants, like 

 the Venus 's flytrap, the 

 sundew (see paragraph 

 138), and the pitcher 

 plants, of which a good 

 example is the com- 

 mon pitcher plant of 

 our sphagnum moors. Here the leaf is modified into a pitcher- 

 shaped structure, broadened near the middle and narrowed 

 somewhat near the free end, where there are on the inside 

 of the pitcher numerous bristle-like hairs pointing downward. 



Fig. 78. 

 Tendrils of sweet pea coiling around supports. 



