THE STEM AND THE LEAF 



71 



-Fi<;. 52. Top view of ver- 

 tical shoot of syringa 

 (Philadelphia) 



The leaves are arranged in 

 pairs, and each pair over- 

 lies the spaces between the 

 pair immediately below it. 

 One fourth natural size 



the fibrnvacular bundles! connecting it with the stem. The im- 

 portance i if tliis is clear from what has already been said 

 ( sect. 37) about photosynthesis as a 

 process of food-making in which the 

 elements of water from the soil and of 

 carbon dioxide from the air are brought 

 together in the leaf to form sugar and 

 starch. 



Have the veins of the leaf other uses 

 besides then- function as conveyers of 

 water ? Explain. 



69. Alternate and opposite arrange- 

 ment of leaves. When a leafy apple 

 twig (fig. 51) is compared with one of 

 maple or box. elder, it is evident that 

 the former has its leaves arranged in a 

 spiral order, while the latter bears its leaves in pairs. One 

 leaf of each pair is on 

 the opposite side of 

 the twig from its mate, 

 and a leaf of each 

 pair covers the inter- 

 val between the two 

 leaves next above or 

 the two leaves next be- 

 low (fig. 52). Leaves 

 borne in spirals are 

 said to be alternate, 

 and those in pairs, 

 like maple leaves, are 

 said to be opposite. 



The spiral arrange- 

 ment is much the 

 commoner, being char- 

 acteristic of most herbs, most shrubs, and very many hard-wood 

 and fruit trees. Some of the most familiar opposite-leaved 



Top view of a horizontal shoot from 

 the shrub shown in figure 62 



The leaves spring from the branch in the same 

 order as do those of the vertical branch, but by 

 a twisting of the leafstalks the blades are made to 

 lie in a nearly horizontal position, and thus secure 

 abundant illumination. One fourth natural size 



