DIV. I 



MORPHOLOGY 



179 



in the one case the new tubers appear to grow out of the side, and in the other 

 to spring from the top of the old tubers. 



The Radish is also a tuberous stem, although only a portion of a single 

 internode, the hypocotyl of the seedling, is involved in the swelling. 



(b) BULBS also belong to the class of subterranean metamorphosed 

 shoots. They represent a shortened shoot with a flattened discoid 

 stem (Fig. 204 zk), the fleshy thickened scale leaves (zs) of which are 

 filled with reserve food material. The aerial shoot of a bulb develops 

 from its axis, while new bulbs are formed from buds (k) in the axils 

 of the scale leaves. 



(c) Other herbaceous perennials of periodically moist climates 

 (e.g. the Dahlia and many Orchids) form ROOT-TUBERS (Figs. 205, 206). 

 They resemble the stem- 

 tubers, though their true 



nature can be recognised 

 by the presence of a root- 

 cap, the absence of leaves, 

 and the internal structure. 

 Tuberous main roots are 

 found in the Carrot and 

 the Beet, both of which are 

 biennial plants. 



The morphology of the 

 tubers of the Orchidaceae is 

 peculiar. They are, to a great 

 extent, made up of a fleshy 

 swollen root terminating above FIG. 206. Root-tuber of Orchis somewhat diagrammatically 

 in a shoot-bud. At their lower represented, t', The old root-tuber ; t", the young root- 



j. AT, j. -u -4.1 tuber ; &, floral shoot ; s, scale leaf with axillary bud, k, 



extremity the tubers are either from ^ ^ new ^ ^ aris(m . ^ * ^ 



simple or palmately segmented. titious ^^ . ttj the scar on the old tuber ^^g its 

 In the adjoining figure (Fig. attachment to its parent shoot, (f nat. size.) 

 206) both an old (f) and a young 



tuber (t"} are represented still united together. The older tuber has produced its 

 flowering shoot (b), and has begun to shrivel ; a bud, formed at the base of the 

 shoot, in the axil of a scale leaf (s), has already developed an adventitious root 

 which has given rise to the younger tuber. Roots of ordinary form arise from 

 the base of the stem above the tuber. 



Many bulbs, tubers, and rhizomes occur at a SPECIFIC DEPTH, 

 which may, however, vary with the nature of the soil. Thus the 

 rhizome of Paris is placed at a depth of 2-5 cm., that of Arum at 

 6-12 cm., of Colchicum at 10-16 cm., and of Asparagus officinalis at 

 20-40 cm. The seeds of these plants germinate close to the surface 

 of the soil so that the subterranean shoots of the young plants must 

 penetrate more and more deeply into the earth. This may be effected 

 by the movements of growth of the stem (cf. p. 345) or by contractile 

 roots. Thus in Lilium (Fig. 207) all the roots are highly contractile ; 

 this is best seen in Fig. 207, 3, where the two lowest roots have con- 



