102 



LEAVES AND FOLIAGE 



155. Different forms of leaves from 

 one plant of ampelopsis. 



leaves of grasses, sedges, lilies, orchids, and other plants 

 known as monocotyledons (Chapter XXV). Most leaves 



of this character are paral- 

 lel-veined. 



217. Leaves also die and 

 fall from lack of light. Ob- 

 serve the yellow and weak 

 leaves in a dense tree-top 

 or in any thicket. Why do 

 the lower leaves die on 

 house-plants? Note the 

 carpet of needles under the 

 pines. All evergreens shed 

 their leaves after a time. 

 Counting back from the tip 

 of a pine or spruce shoot, 

 determine how many years 

 the leaves persist. (Fig. 158.) In some spruces a few leaves 

 may be found on branches ten or more years old. 



218. Although the forms and positions of leaves often 

 have direct relation to the places and conditions in which 

 the leaves grow, it is not probable that all forms and shapes 

 have been developed to adapt the plant to its environment. 

 It is probable that the toothing or lobing of the leaf-margins 

 is due to the same causes that produce compounding or 

 branching of leaves, but 

 what these causes are is 

 not known. It has been 

 suggested that leaves have 

 become compound in order 

 to increase their surface and 

 thereby to offer a greater 

 exposure to light in shady places, but very many sun-loving 

 species have compound leaves, and many shade-loving 

 species have simple and even small leaves. Again, it has 



156. Muskmelon seedlings, with the unlike 

 seed-leaves and true 



