THE STEM AND THE LEAF 



Tl 



tical shoot 



(Philadelphus) 



The leaves are arranged in 

 pairs, and each pair over- 

 lies the spaces between the 

 pair immediately below it. 

 One fourth natural size 



the fibrovascular bundles connecting it with the stem. The im- 

 portance of this 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 



FIG. 52. Topviewofver- besides their f^ction-as conveyers of 

 of syringa 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- a twisting of the leafstalks the blades are made to 

 ment is much the ^ e * n a near ^y horizontal position, and thus secure 



abundant illumination. One fourth natural size 



commoner, being char- 

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

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



FIG. 53. Top view of a horizontal shoot from 

 the shrub shown in figure 52 



The leaves spring from the branch in the same 

 oi-der as do those of the vertical branch, but by 



