i62 METABOLISM 



This is the case with those perennials which lose all their aerial parts in winter, 

 and in trees which, at least in many cases, cast off their leaves in the winter season. 

 Furthermore, reserves develop in assimilatory organs themselves when the 

 assimilatory products are formed more rapidly than they are used up or 

 removed. In all cases, however, before the reserves are actually employed they 

 must be chemically altered and rendered mobile. This we have yet to consider, 

 although we may dismiss the subject in a few words since the methods 

 employed are fundamentally like those already referred to in seeds. 



The reservoirs for reserves in perennials are in the form of aggregations of 

 large-celled storage parenchyma situated in the interior of the plant, and often 

 indicated externally by conspicuous swellings. The storage tissue may occur 

 in the root, in the hypocotyl, in the stem, or in the leaf, and hence we are led, 

 from the morphological point of view, to distinguish such swellings as root 

 tubers, stem tubers and bulbs. In close relation to such storage tissues we 

 find one or more buds which are capable of developing into shoots in the 

 following year. The reserves present in these bodies are on the whole the 

 same as those found in seeds, and consist of nitrogenous and non-nitrogenous 

 organic substances as well as constituents of the ash, which we need not con- 

 sider further. In one point only seeds differ from subterranean storage organs, 

 viz. that when ripe they are for the most part in a desiccated condition, and 

 the absorption of water is the essential preliminary to their germination. Sub- 

 terranean reserve organs, on the other hand, always contain a considerable 

 percentage of water ; if they be artificially deprived of water to the same extent 

 as seeds are, they would in most cases soon come to grief. It is well known, for 

 instance, that potato tubers are able to develop shoots if not in too dry an 

 atmosphere, owing to their possessing a store of water, and that some bulbs and 

 tubers may give rise to shoots bearing flowers, without any absorption of water 

 being observable. Medicus (1803) noted this in the case of Veltheimia 

 capensis ; Hildebrand (1884) drew attention to the same fact in Oxalis 

 lasiandra, and recently Sanromatum guttatum has been put on the market as 

 a curiosity in consequence of its power of forming flowers in the complete 

 absence of water, after being heated. We may indeed say that water itself 

 is, in many subterranean storage regions, a reserve substance. 



Amongst non-nitrogenous reserves, carbohydrates are entitled to the first 

 place for they are even more abundant in these underground storage organs 

 than they are in seeds. On the other hand, fats, so common in seeds, occur but 

 rarely in subterranean storehouses (e. g. Cyperus escidentus). The carbohy- 

 drate present is very frequently starch, although we often find present in 

 addition, or exclusively, substances which we have not mentioned in speaking 

 of seeds, because they are either absent or are of secondary importance. Such 

 substances are mucilage and varieties of sugar. Mucilage as a reserve may be 

 found abundantly in the tubers of Orchidaceae and in the rhizome of Sym- 

 phytum (compare Frank, 1866). [Mucilage corresponds to reserve cellulose 

 in these situations and gives, on hydrolysis, mannose and galactose. Herissey, 

 1903.] Among the forms of sugar glucoses occasionally occur as reserves, e. g. 

 in the onion. These sugars are capable of being used in the plant for other 

 purposes without further transformation ; since, however, their accumulation 

 must obviously cause an increase in osmotic pressure, we may conclude that the 

 plant for the most part unites several molecules of glucose into a molecule of 

 larger size with separation of water. Thus, for example, the transformation 

 of glucose into cane sugar reduces the osmotic pressure by one-half, and it will 

 be further reduced if substances such as inulin are formed, which have a com- 

 position similar to that of starch, but which remain in solution in the cell-sap. 

 Inulin occurs as a reserve especially in the Compositae and Campanulaceae ; 

 a substance at least very like it in composition is found also in certain Liliaceae. 

 ■Cane sugar is the dominant reserve in sugar-beet and also in the sugar-cane 



