SPECIAL FORMS. 



59 



species, and in older individuals, it is longer, often oblique, and 



branching, and bears the scars from which the annual aerial 



growths have separated. 1 Nymphaea odorata, 



the sweet-scented white Water Lily, grows by 



very long, stout, and simple rootstocks. In 



N. tuberosa the sides of the rootstock produce 



short lateral branches or tubers. 



117. A Tuber may be morphologically char- 

 'acterized as a short thickened rhizoma on a 

 slender base, or a rootstock some portion of 

 which mostly a terminal portion and involv- 

 ing several nodes is thickened by the depo- 

 sition of nourishing matter. A potato and a 

 Jerusalem artichoke are typical examples 

 (Fig. 104-107) ; and the diiference between 

 these subterranean branches and the roots which they may bear 

 is very obvious. Their eyes are axillary buds ; the leaves which 

 subtend them are plainly dis- m 

 cernible, in the form of short and 

 closely appressed scales. In the 

 attempt, occasionally seen, to 

 form axillary tubers above- 

 ground by the Potato-plant, the 

 leafy nature of the scales is 

 evidenced. (Fig. 

 105.) By heaping 

 the soil around 

 the stems, the- 

 number of tuber- \ 

 iferous branches 

 may be in- 

 creased. The number of nodes and internodes involved in a 

 tuber may be many or few. There is one instance of what may 



in autumn, to be renewed by an axillary bud, which makes its subterranean 

 growth and the rudiments of the aerial in early summer. 



1 This rhizoma is a monopodium, being continued year by year by the 

 terminal bud, and the aerial stem or stems sent up in spring, bearing the 

 whorl of leaves and blossom, are axillary branches. 



FIG. 103. An older and longer one of the same species, showing branches, scars left 

 by former leaf- and flower-bearing stems : also at tip (stripped of the covering scales), the 

 bases of two such stems of the season, and the terminal bud between them, for the con- 

 tinuation of the growth of the rootstock, &c., the next season. 



FIG. 104. Base of stem of Helianthus tuberosus, or Jerusalem Artichoke, developed 

 from a tuber, and producing a second generation of tubers. 



FIG. 105. Monstrosity of a Potato-plant, with an axillary bud developing into some- 

 thing between a bulblet and a tuber ; the scales represented by obvious leaves. (From 

 the Gardeners' Chronicle.) 



