THE PLANT-CELL 



55 



grains are completely imbedded in the starch-forming leucoplast, and 

 grow equally on all sides, while the larger oval ones become free on 

 one side, which ceases to grow, while new material is only deposited 

 on the side in contact with the leucoplast. The larger starch-grains 

 generally present a distinctly laminated appearance, due to layers of 

 different density, and there is a small spot, the hiluni, about which the 

 lamellae are arranged concentrically. Good examples of such starch- 

 grains are offered by the Potato, species of Canna, and the spores of 

 Marsilia. Compound starch-grains, such as those in oatmeal, are 

 not uncommon, and in species of Euphorbia they are dumb-bell 

 shaped. Chemically, starch is closely related to cellulose and 

 sugars like glucose, into which it is easily converted. The chemical 

 formula is the same as that of cellulose. 1 



Albuminous Granules. The granules of reserve-food may often be 

 of albuminous nature, i.e. they contain nitrogen, and differ much less 



B 



FIG. 36. A, cell from the stem of a Begonia containing crystals of calcium-oxalate 

 (X 250). B, separate crystals from the same plant. C, cell from the ovary of 

 Sparganium eurycarpum, with needle-shaped crystals, or rhaphides (X 250). 



from the living proteids than do the starch-granules. These albumi- 

 nous bodies may be roundish or irregular grains, as in the gluten- 

 cells of the wheat-grain (" Aleurone-grains "), or sometimes they 

 assume a crystalline form. Such protein-crystals occur in many 

 seeds, e.g. the Brazil-nut (Bertholletia excelsa) and the Bur-reed 

 (Sparganium). They may also be found in the cortical cells of the 

 potato-tuber. The protein-crystals, or " crystalloids," as they are 

 often called, may be found in all parts of the cell, even within the 

 nucleus. 



Oil. In some plants the starch is partly or entirely replaced by 

 fatty oil. Thus in the common Alga Vaucheria, oil replaces the 

 starch as the first visible product of photo-synthesis. In many 



1 See also Zimmermann (p. 225) for a discussion of substances related to starch. 



