THE METABOLISM OF PLANTS. 21 7 



organic acids, but as our knowledge of their significance is 

 at present so incomplete, it will be better to classify them for 

 the present with the non-nitrogenous waste-products. 



With regard to the formation of the carbohydrates in 

 the plant we have but little definite information. Of glucose 

 we may say that it is derived by the action of an unorganised 

 ferment from one or other of the carbohydrates, which, as we 

 have seen (p. 170), are stored up as reserve-materials, except 

 in certain plants, the Onion for example (Sachs), in which it 

 appears to be formed in the chlorophyll-corpuscles in the. first 

 instance. Of the reserve-carbohydrates, all that we can say 

 is that they are derived, in the case of green plants, from 

 the non-nitrogenous organic substance, whatever it may be, 

 which is formed in the chlorophyll-corpuscles under the in- 

 fluence of light, and, in the case of plants destitute of chloro- 

 phyll, from the organic substances supplied to them as food : 

 but the only definite information which we possess as to the 

 mode of this derivation is confined to starch and cellulose. 



It has been already stated (p. 180), upon the authority of 

 the observations of Schimper and of Strasburger, that the 

 starch which makes its appearance in chlorophyll-corpuscles 

 or in amyloplasts is formed from protoplasm ; that mole- 

 cules of protoplasm undergo dissociation, and that starch is 

 a conspicuous product of the process. In support of this 

 statement we may now adduce some additional evidence. 

 The cells of the embryo of the Wheat contain, when the seed 

 is quiescent, no trace of starch, but Just has observed that 

 if a seed be kept moist for twenty- four hours, the embryo 

 then contains starch abundantly, although the endosperm 

 has apparently undergone no change, and no trace of sugar 

 can be detected in it. It is difficult to account for this fact 

 in any other way than that suggested by Just, namely, that 

 the starch is formed by dissociation from the protoplasm in 

 the cells of the embryo. 



It has also been stated, upon the authority of Schmitz 

 and of Strasburger, that cellulose is likewise formed by dis- 

 sociation from protoplasm. In support of this statement it 

 may now be pointed out that Schmitz has observed that 



