162 



REVIEW OF THE PRINCIPLES OP NUTRITION. 



exact elements of water, viz., C12 Hio Oio. Then, through the action of light, chlo~ 

 rophylle springs into being, clothing the plant in living greea Meanwhile 



853. Gun, starch and sogar, nutritive products common to all plants, are also 

 developed from the proper juiet3 — not all to be immediately employed in building up 

 the tissues, but mostly to be stowed away in reserve for future use. Such deposits 

 are made in the root of tho beet, tuber of the potato, and in the fruit generally. 

 These three products, with cellulose, are all composed of carbon and tho elements 

 of water, often in identical proportions ; thus cane sugar is C12 H12 O12 ; grapo sugar, 

 On H11 On ; gum, C12 Hio Oio ; starch, C12 IT10 Oio ; cellulose, C ]2 Hio Oio. 



854. Sugar is sometimes produced directly from the proper juice, as in tho 

 root of beet, stalk of maize, and sugar-cane ; but oftener, during germination, from 

 the starch deposited in tho seed. Its composition, as seen above, differs from that 

 of starch only in containing a larger proportion of tho elements of water or (what 

 is the same) a smaller proportion of carbon. As starch i3 insoluble, its transforma- 

 tion into soluble gum or sugar is needful to render it availabta for tho nutrition of 

 the growing embryo. 



855. The facility with which these five general products are con- 

 verted into each other, both in the growing plant and in the laboratory of tho 

 chemist, is accounted for by the similarity of their chemical condition. Thus starch, 

 gum and cellulose may reconvert merely by some change in tho arrangement of their 

 constituent atoms, or they may becomo sugar by the addition of ono or two atoms 

 of water. 



856. Among the numerous secretions of plants, which our limits forbid us to 

 consider, are tho vegetable acids containing more oxygen proportionately than 

 exists in water ; the oily acids, resins and oils, containing less oxygen than exista 

 in water, or none at all. These substances vary in the different species almost to 

 infinity, taking into their constitution, in addition to tho four organogens, minuto 

 portions of the mineral substances introduced by rain and river water. Their pecu- 

 liarities of odor, flavor, color, properties, etc., although so obvious to the senses, are 

 occasioned by differences of constitution often so slight as to elude the most delicate 

 tests of tho chemist. 



857. The following table contains examples of tho various classes of secre- 

 tions, arranged in reference to their relative proportion of oxygen : — 



