CHAPTER II LEAVES 



i. CHLOROPHYLL AND FOOD MANUFACTURE 



Introductory statement. Leaves usually possess a more or less ex- 

 panded portion, the blade, which mayor may not be borne on a stalk, 

 the petiole. The blade is composed of veins and of the green parts 

 between the veins, the mesophyll; the latter is the seat of food manu- 

 facture, and the former are organs of support and transportation. In 

 many cases leaves are scalelike and take no part in food making, 

 while, on the other hand, stems frequently have an important part in 

 this process. The chief foods manufactured by plants are carbohydrates 

 (such as the various sugars and starches), fats, and proteins (such as 

 the albumins). The simplest of 

 these foods, the carbohydrates, are 

 manufactured first, and most of 

 our knowledge of food making 

 deals with this synthesis of car- 

 bohydrates. 



Chloroplasts and chlorophyll. FIGS. 753, 754. Chloroplasts: 753,3 



The Chloroplasts. The synthesis cel1 from a moss leaf > sh win 8 



. , , , . i . , plasts in various stages of division; the 



of carbohydrates is associated with bodies wfthin the chloroplasts are starch 



the green bodies of plant cells, grains; 754, a cross section of a leaf of 



the Chloroplasts, which consist of Selaginella Martensii, showing two kinds of 



chloroplasts, those in the lower part of the 



a colorless, protoplasmic, sponge- leaf being small and resembling the chloro . 



like matrix, the Stroma, suffused plasts of 753, while those in the upper 



Or impregnated, at least in the epidermis are large and solitary; note the 



. . irregular shape of the latter, some being 



peripheral portions, with a green more or lesg mortar . shap ed, the maximum 



pigment, the chlorophyll. In shape, surface exposure being toward the light; 



chloroplasts generally are some- 



note also the thinness of the leaf (three cells 

 , f thick); both figures highly magnified. 



what spheroidal or ellipsoidal (or 



even polygonal, if crowded), the number in a cell varying from few to 

 many (fig. 753; also fig. 758). However, in the Conjugales and in many 

 other algae there are one to several large chloroplasts in a cell, which 

 may be. tabular, spiral, cylindrical, cuplike, or stellate in shape (fig. 106). 



521 



