CHAPTER IV 



The Non-livixg Contents of Cells 



Apart from the living constituents, cells usually contain numerous 

 substances which are either dissolved in the sap or, when in- 

 soluble, occur as solid bodies or suspended drops. These sub- 

 stances can for the most part be grouped as food-bodies or as 

 by-products, according as they are known to be employed in 

 the nutrition of the plant or arc believed to be of no further 

 nutritive value. 



Among the commoner food-substances are ^'arious carbo- 

 hydrates {e.g. sugars, starch, etc.), oils, and proteins, all of which 

 are built up by the plant from simple inorganic compounds by a 

 scries of complex changes. Durmg the earlier part of the season 

 such food-substances are used directly to supply the necessary ma- 

 terials for growth, but subscquentl}'-, with decreasing demands, 

 a large proportion arc stored up for future use. In perennial 

 plants they accumulate in those organs which persist from year 

 to year {e.g. bulbs, tubers, etc., and, in the case of woody per- 

 ennials, the stem- and root-systems).'- Similar food-substances 

 are, moreover, laid up within the cotyledons or endosperm of 

 all seeds. 



One of the most important food-substances is stairli, which 

 is insoluble in the cell-sap. It is often the first easily recognisable 

 product of photosynthesis in a green leaf which has been exposed 

 to light. On microscopic examination the starch appears as very 

 small shining grains, mainly within the chloroplasts (Fig. ], St.). 

 These grains gradually increase in amount during the day, but 

 generally disappear over-night, and are consequently spoken of 

 as transitory starcli. Their gradual accumulation on a bright day 



^ Cf. 1''. cv S., chapter xii. 

 40 



