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. 

 The chloroplasts. The synthesis 

 of carbohydrates is associated with 

 the green bodies of plant cells, 

 the chloroplasts, which consist of 

 a colorless, protoplasmic, sponge- 

 like matrix, the stroma, suffused 

 or impregnated, at least in the 

 peripheral portions, with a green 

 pigment, the chlorophyll. In shape, 

 chloroplasts generally are some- 

 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 Conjugates 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 



753 754 



FIGS. 753, 754. Chloroplasts: 753, a 

 cell from a moss leaf, showing chloro- 

 plasts in various stages of division; the 

 bodies within the chloroplasts are starch 

 grains; 754, a cross section of a leaf of 

 Selaginella Martensii, showing two kinds of 

 chloroplasts, those in the lower part of the 

 leaf being small and resembling the chloro- 

 plasts of 753, while those in the upper 

 epidermis are large and solitary; note the 

 irregular shape of the latter, some being 

 more or less mortar-shaped, the maximum 

 surface exposure being toward the light; 

 note also the thinness of the leaf (three cells 

 thick); both figures highly magnified. 



