CHAPTER IV 



THE NON-LIVING 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 are believed to be of no further 

 nutritive value. 



Among the commoner food-substances are various 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 

 series of complex changes. During the earlier part of the season 

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

 terials for growth, but subsequently, with decreasing demands, 

 a large proportion are 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). 1 Similar food-substances 

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

 all seeds. 



One of the most important food-substances is starch, 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. 3, St.). 

 These grains gradually increase in amount during the day, but 

 generally disappear over-night, and are consequently spoken of 

 as transitory starch. Their gradual accumulation on a bright day 



1 Cf. F. & S., chapter xii. 

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