82 



FOUNDATIONS OF BIOLOGY 



omy in one of the main divisions (Dicotyledons) of Flower- 

 ing Plants. (Fig. 45.) 



3. Leaf 



The embryonic cells forming the growing point, or bud, of 

 the shoot comprise, as we have seen, the fundamentals of 

 both stem and leaves; that is to say, stem and leaves arise 

 together in buds. The method of formation of stem and 

 leaves is well seen in the buds of a common water plant, 



FIG. 46. A bud, of unusually elongated form, of Elodea canadensis, in 

 exterior view and section, showing the development of leaves; X 150. 

 (From Ganong, after Kny.) 



Elodea. Here the rounded end of the stem is composed of 

 the characteristic embryonic tissue. The rudiment of each 

 individual leaf is first visible on the surface as an enlarged cell 

 which by division and differentiation gradually develops into 

 a flat projection of epidermal, cortical, and vascular tissue, 

 constituting the fully formed leaf. (Fig. 46.) 



Bearing in mind that the leaf is the chief organ for the in- 

 take and utilization of the energy of sunlight and for the 

 interchange of gases, it is evident that it forms the second of 

 the two chief points of contact of the plant with its sur- 

 roundings. This intimate relationship is manifest in the in- 



