256 Plant Physiology 



juices of selected races and strains of the two plants indi- 

 cated may contain from 14 to 18 per cent of sugar, and the 

 history of the breeding of the highly productive races of 

 the sugar-beet is of special physiological interest. Sor- 

 ghum and a few species of tropical palms also contain 

 cane-sugar in sufficient quantity to be of commercial im- 

 portance. Sugar maple (Acer saccharum) growing in 

 northern latitudes yields a sap which in the usual time of 

 cupping (late winter or early spring) may contain from 

 2 to 5 per cent of cane-sugar, besides other substances 

 imparting the peculiar flavor for which this sugar is prized. 



144. Starches. The great majority of green plants 

 produce starch (Fig. 65). There are some exceptions 

 among several orders of flowering plants, especially certain 

 monocotyledons (Sect. 112) ; and certain groups of algse 

 do not possess this capacity, notably the families of blue- 

 greens and browns. 



The starch molecule is very complex and difficult of ex- 

 act study, but the occurrence and reaction of the starches 

 are well known. Starch occurs in the form of insoluble 

 grains with a characteristic general appearance, varying 

 considerably, however, in form, size, and markings in the 

 different plants in which produced. The grains may be 

 simple, semicompound, or compound. Very large grains, 

 often so large as to be visible to the unaided eye, are found 

 in the root-stock of Canna ; those of potato are of medium 

 size ; in rice the components of the compound grains are 

 small and numerous ; while in spinach they may be ex- 

 tremely minute, and according to Nageli as many as 30,000 

 may be united together. 



Starch grains are produced within plastids, chloro- 



