THE ANGIOSPERMAE : STEMS 



937 



leaves, and become centres of new growth when the rhizome connecting 

 them to the parent plant has perished by decay. Those of Cyperiis escu/entus, 

 Stachys tuberifera (Fig. 923) and oi Sagittaria sagittifolia are edible. 



Branch tubers may be typified by the Potato. The tuber-bearing branches 

 come from the lowest axils of the aerial stem, even, in seedlings, from the 

 cotyledonary axils. These branches are plagiotropic or weakly geotropic, 

 and if formed above ground they bury themselves and swell out at their 

 apices to form the massive tubers, which bear temporary scale-leaves with 

 buds in their axils called " eyes." The tuber has a corky periderm and 



i— Axillary branch 



Leaf scar 



Fig. 924. — Solanum tuberosum. Portion of an 

 aerial shoot bearing axillary tubers. 



may remain dormant for some time. When it germinates, orthotropic shoots 

 are formed from the axillarv buds, which become new aerial stems. Even 

 a single detached bud will grow and form a new plant. In some conditions 

 axillary buds near the top of the leafy shoots may be transformed into sessile 

 tubers (Fig. 924). Placing the plant in the dark under a box will often 

 stimulate this abnormal tuber formation. The tubers of the Jerusalem 

 Artichoke {Heliatithus tuberosiis) (Fig. 925) are formed like those of the 

 Potato, but they are borne on shorter branches, are often themselves branched 

 and have persistent, fleshy scale leaves. They contain inulin instead of 

 starch. Similar but more slender tubers are formed by the perennial 

 Sunflower [Helianthus rigidus). 



