454 CONSTRUCTIVE AND DESTRUCTIVE METABOLISM 



absent. Changes of this character do actually take place in the cell-wall, 

 for either its original constituents may undergo chemical change, or new 

 substances may be introduced, or both these modifications may take place 

 simultaneously (Sect. 84). 



Nitrogenous and non-nitrogenous reserve-materials have hitherto 

 always been found associated together. This is not, however, necessarily 

 essential, for plants can grow when sugar is the sole carbon-compound sup- 

 plied to them, and when all the other essential elements, including nitrogen, 

 are absorbed in the form of inorganic salts. Fungi may grow when proteid- 

 substances afford the sole source of carbon, in which case nitrogenous 

 compounds must be excreted as one of the end-products of metabolism. 

 In seeds, the presence of non-nitrogenous carbon-compounds renders possible 

 a more economic consumption of those which are nitrogenous, so that a loss 

 of nitrogen is avoided as a general rule. 



The various changes which the stored substances may undergo suffice 

 as proof that no special respiratory or cell-wall-forming substances exist, 

 nor are particular food -materials used solely in respiration or solely in 

 cell-wall formation, germination, &c., although it is always possible that, 

 when presented simultaneously, one carbon compound may be mainly used 

 in respiration, another mainly in proteid-synthesis. Owing to the pre- 

 paratory changes the food may undergo when absorbed, the final stages 

 of a particular metabolic activity may be identical whatever the food 

 may be (Sects. 66, 77). 



The use to which a formative substance is put determines its localiza- 

 tion, and the distribution of plastic substances in the different plant-organs 

 has already been discussed in connexion with translocation (Chap. X). 

 With the exception of reserve-cellulose, reserve-materials are stored 

 within living protoplasts, partly in the vacuoles and partly in the plasma 

 itself. In turgid cells the dissolved substances are present in solution, 

 starch in granular form, oil either as visible drops or in the form of an 

 emulsion so fine as to be visible only when the droplets are caused to 

 coalesce. 



Soluble carbohydrates, amides, proteids, organic acids, &c. are pre- 

 sumably mainly dissolved in the cell-sap of the vacuoles, in which a large 

 amount of oil may also be present, whereas starch-grains appear normally 

 to remain imbedded in the protoplasm where they were formed. The 

 stored substances present in the cell-sap or elsewhere are reassimilated 

 by the protoplasm when necessary, and they must always pass through 

 the latter to reach those regions where they are required. Vacuoles 

 are important organs which are of service in many ways, for example, 



1 Cf. Hofmeister, Pflaiizenzelle, 1867, p. a. 



