48 



FORMS OF STEMS. 



The ordinary herbaceous stem of the potato, when cut into slips and 

 planted, sometimes sends off branches from its base, which assume the 



Fig. 108. 



Fig. 109. 



form of tubers. These tubers occasionally become nodulated, or elon- 

 gated, or curved in various ways. Arrow-root is derived from the 

 scaly tubers of Maranta anmdinacea. In the Orchis the radicular 

 bodies called tubercules, or by some tubers, 

 belong to the root system (fig. 101). In the 

 didymous (twin) tubers of Orchis mascula, 

 we find at the end of the season one of 

 them withered, while the other is vigorous, 

 and bears a bud at its apex. The lowest 

 leaf of this bud gives rise to another bud, 

 and when the oldest tuber decays this 

 new one enlarges, and next season be- 

 comes the bud-tuber, while its parent pro- 

 duces the flowering stem. A Corm is a 

 solid underground stem which does not 

 spread by sending out shoots, but remains 

 of a rounded form, and is covered by thin 

 scales on the outside (fig. 110). The scales 

 are modified leaves specially developed 

 on subterraneous stems, and they may 

 The corm occurs in Colchicum, Orocus, and 



Fig. 110. 



produce buds in their axils, 



Fig. 108. Soboles, or creeping subterranean stem, r, of Scirpus lacustris. /e, fe. Scales 

 or modified leaves on the stem, p a. Aerial portion of the plant. 1 1, Level of the fearth. 

 Fig. 109. Lower portion of a potato plant, s s. Level of earth, pa, pa, Aerial portion 

 bearing leaves, t, Subterranean portion, showing stem-tubers. T, Tuber showing eyes or leaf- 

 buda, covered by scales, 6, which are equivalent to leaves. Fig. 110. Corm or under- 

 ground stem of Colchicum autumnale. r. Roots. /, Leaf, a'. Ascending axis of preceding 

 year, withered, a", Axis of the year, a'". Point where axis of next year would be formed. 



