MORPHOLOGY 



23 



two preceding years ; and at b may- be seen the base of the stem 

 growing at the time the rhizome was taken from the ground, while at 

 a is shown the bud of the next year's aerial growth. The rhizome of 

 Coralliorrhiza innata, a saprophytic Orchid, affords a good example of a 

 root-stock functioning as a root (Fig. 22). Bulbs, also, belong to the 

 class of metamorphosed shoots. They represent a shortened shoot 

 with a flattened, discoid stem (Fig. 23, zk), the fleshy thickened scale 



Fig. 22.— Khizome of Coralliorrliiza innaia. 

 a, Floral shoot ; &, rudiments of new 

 rhizome branches. (After Schacht, 

 nat. size.) 



Fio. 23. — Longitudinal section of tulip 

 bulb, Tulip". Gesneriana. zk, Modified 

 stem ; rs, scale leaves ; v, terminal 

 bud ; k, rudiment of a young bulb ; v: , 

 roots. (Nat. size.) 



leaves (zs) of which are filled with reserve food material. The aerial 

 growth of a bulb develops from its axis, while new bulbs are formed 

 from buds (k) in the axils of the scale leaves. Another form of 

 underground shoot, allied to bulbs and connected with them by 

 transitional forms, is distinguished as a tuber. The axis of a typical 

 tuber, in contrast to that of a bulb, is fleshy and swollen, functioning 

 as a reservoir of reserve material, while the leaves are thin and scaly. 

 Of such tubers those of the Meadow Saffron (Colchicum autmnnah) or 

 of Crocus satims are good examples. In the Meadow Saffron new 

 tubers arise from axillary buds near the base of the modified shoot, 

 but in the Crocus from buds near the apex. In consequence of this, 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 tubers of the Potato 



