no 



BOTANY OF THE LIVING PLANT 



starch or cellulose, or it may be transformed into fats. In order to 

 be again of physiological use these insoluble substances must be con- 

 verted back into sugar. The methods of storage and transfer of carbo- 

 hydrates may best be illustrated by the commonest example, viz. by 

 the case of Sugar and Starch. 



m^ 



9 



Fig. 79. 

 Cells of young Potato. A, with minute leukoplasts surrounding the nucleus. 

 B shows some of these already forming starch, stained darkly with iodine. C shows 

 further starch-formation, and one cubical protein-cr>-stalloid. ( x 220.) 



Sugar is a comprehensive term which covers a number of closely 

 related carbohydrates soluble in water. Glucose is probably the first 

 product of Photo-Synthesis. Sugar is also the usual form in which 

 carbohydrate may be transferred from place to place in the plant. 

 This depends upon its solubility in water, through which medium it 



may move by diffusion. The 

 tissues contain water of imbi- 

 bition, and also fluid water ; 

 and though the living proto- 

 plasts are able to exercise 

 control over movements by 

 diffusion, nevertheless water as 

 the ready medium of deposit, or 

 of transit for sugar is constantly 

 present. But soluble sugar is 

 a bulky form of accumulation 

 of carbohydrate, and being 

 osmotically active, would in large quantities raise difficulties in rela- 

 tion to turgor of the cells. Consequently it is commonly converted 

 into insoluble Starch, in the form of Starch-Grains. Such grains are 

 not formed at large in the cell, but always in relation to plastids. 

 These may be in the form of green chloroplasts, in which the included 

 starch-grains make their appearance as the first visible result of Photo- 



FlG. 80. 



A, cell from the endosperm of Ricinus in water. 

 B, isolated aleurone grains in oil. ft = albumen 

 crystals. g=globoid. (X540.) (After Strasburger.) 



