162 



REVIEW OF THE PRINCIPLES OF NUTRITION. 



exact elements of water, viz., Ci 2 HI O Oio. Then, through the action of light, chlo- 

 rophylle springs into being, clothing the plant in living green. Meanwhile 



853. GUM, STARCH AND SUGAR, nutritive products common to all plants, are also 

 developed from the proper juice 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 the beet, tuber of the potato, and in the fruit generally. 

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

 of water, often in identical proportions ; thus cane sugar is Ci 2 His Oia ; grape sugar. 

 Ci-> Hn On ; gum, Ci 2 HIO Oio; starch, On Hio Oio; cellulose, C J2 HIO Oio. 



854. SUGAR is SOMETIMES PRODUCED DIRECTLY from the proper juic3, aa in the 

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

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

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

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

 tion into soluble gum or sugar is needful to render it available for the nutrition of 

 the growing embryo. 



855. THE FACILITY WITH WHICH THESE FIVE GENERAL PRODUCTS ARE COX- 

 VERTED IXTO 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 the arrangement of their 

 constituent atoms, or they may become 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 the vegetable acids containing more oxygen proportionately than 

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

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

 infinity, taking into their constitution, in addition to the four organogens, minute 

 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 the chemist. 



857. THE FOLLOWING TABLE CONTAINS examples of the various classes of secre- 

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



