THE ANGIOSPERMAE : STEMS 939 



Many members of the Dioscoreaceae form stem tubers. In Tamus com- 

 munis (Black Bryony) the first internode above the cotyledons thickens to 

 form a perennial tuber, which may reach a very large size and may branch. 

 From this tuber the leafy shoots are annually renewed. Another well-known 

 example is Testudinaria elepluvitipes, a South African desert plant (Fig. 927), 

 in which the base of the stem forms a large tuber, as much as 30 cm. across, 

 which stands partly above ground. It is covered by a very thick, irregular 

 periderm which gives it the name of Elephant's Foot. As in Tamus, delicate 



Fig. 927. — Testudinaria elephantipes. A large woody tuber 

 from which the annual foliage shoots arise. 



or trailing shoots are produced from the tuber each spring and disappear in 

 the dry season. 



4. Corms. The corm is simply a special form of underground tuber, 

 consisting of a much contracted, swollen, main stem, whose principal axis is 

 vertical. On top it bears an apical bud, from which summer shoots with 

 leaves and flowers are produced, and it bears an annual crop of adventitious 

 roots, usually from the lower end (Fig. 928). Good examples are provided 

 by Crocus and Gladiolus. Here the corm is considerably flattened, its length 

 being shorter than its diameter. It is covered by a tunic made up of the 

 fibrous remains of leaf bases, which arise from nodes that form a series of 

 ridges around the corm. At the nodes there are occasional buds, axillary to 



