470 CONSTRUCTIVE AND DESTRUCTIVE METABOLISM 



to further metabolism, so that usually the amount present increases as the 

 plants grow older. Most gums, and also a few forms of reserve-cellulose, 

 are pentosans, that is, compounds of pentose with other substances. Com- 

 bined pentoses may serve as plastic material, and it is even possible that 

 pentoses may normally, or only under special circumstances, be reassimi- 

 lated. This possibility is not entirely due to the nutritive value of the 

 substances in question, for the plant is able when necessary to protect starch, 

 grape-sugar, &c., from immediate consumption (Sects. 77 and 78). 



Many pentoses (arabinose, &c.) constitute a moderately good nutritive 

 medium for mould-fungi, and certain forms of mucilage which may serve 

 as reserve ,/ood-material in the tubers of Orchis, &c. may belong to the 

 group of pentosans. In other plants the mucilaginous substances seem to 

 be permanently aplastic products of merely biological importance, as is the 

 case with regard to those forms of mucilage which dissolve or swell in 

 water. These for the most part appear to be carbohydrates. The in- 

 sufficiently known pectic compounds which are present in part in the cell- 

 wall, and in part dissolved within the cell, apparently belong to the same 

 group of substances, and probably subserve a variety of physiological uses. 



The fatty oils found in plants mainly consist of glycerides of oleic, 

 palmitic, and stearic acids, and during translocation large quantities of 

 free fatty acid are usually present. The oil globules found in the cells 

 of many of the Hepaticae are peculiar fatty masses, which behave as aplastic 

 bodies 1 . The waxy deposits or impregnations which modify the permea- 

 bility of the cell-wall are also aplastic products, although they may be used 

 as food-material by certain fungi 2 . In some cases the waxy substances are 

 esters of monatomic alcohols. 



The different carbohydrates and fats have each their own specific 

 nutritive value, as is shown by experiments with fungi (Sects. 66 and 67), 

 and by the restriction of particular fermentative organisms to certain forms 

 of sugar. The same differences probably exist as regards their nutritive 

 value for higher plants, so far as these require external supplies of non- 

 nitrogenous organic food 3 . It is evident that carbohydrates and fats exhibit 

 a certain amount of physiological equivalence in normal metabolism, for 

 both may serve as reserve-material in the same plant, and in certain cases 

 inulin, starch, grape-sugar, oil, &c., are formed in succession from the same 

 food-substance. 



These and other metamorphoses serve as means to render possible 

 storage, translocation, mobilization, or to prevent loss by exosmosis, 



1 Pfeffer, Flora, 1874, p. 40; \V. v. Kiistcr, Oelkbrper d. Lebermoose, 1894. 

 * R. H. Schmidt, Flora, 1891, p. 315. 



s The formation of starch grains can be induced by the presence of certain substances only 

 (Sect. 55). 



