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AGRICULTURE HANDBOOK 495, U.S. DEPT. OF AGRICULTURE 



located vertically in the soil and 

 usually descend. Unusual varieties 

 are known in which, after growing 

 down awhile, the tip of the tuber 

 begins to grow upward, producing 

 a long, thin, U-shaped tuber. 

 Tubers often enlarge as they de- 

 scend to form a pyramid, but they 

 may also be constricted later in a 

 spindle shape. Tubers of some va- 

 rieties are extremely smooth, while 

 those of others are extremely ir- 

 regular. Shape is complicated by 

 tuber branching, which may occur 

 during early or late growth. There 

 may be few or many branches, 

 either the same size as the main 

 tuber or smaller, and the branches 

 may themselves continue to 

 branch. Some of the most unusual 

 shapes are found among the New 

 Guinea yams, in which the tuber 

 can occur as a series of vertical in- 

 tersecting plants. 



The tubers are renewed annual- 

 ly. A tuber produced one year gives 

 rise in the next to a shoot, the 

 growth of which is usually pre- 

 ceded by the appearance of the 

 primary nodal complex from which 

 roots, new tuber, and stem arise. 

 The new tuber remains extremely 

 small during the initial months of 

 growth, but develops rapidly near 

 the end of the growing season. 



Many varieties also produce 

 single or multiple aerial tubers 

 (fig. 4) in the axils of the leaves 

 at the same time underground 

 tubers are being formed. Aerial 

 tubers often resemble under- 

 ground tubers in shape and 

 branching habit. When they touch 



damp soil, they grow into it rapid- 

 ly. In this way a single plant can 

 give rise to many small, rooted 

 tubers in a short time. Aerial 

 tubers continue to be produced 

 until foliage dies back at the end 

 of the growing season. All are 

 edible, but the smaller ones are 

 immature and inconvenient to 

 prepare for cooking. 



Varieties are distinct with re- 

 spect to flesh color. Anthocyanins 

 color some flesh purple, usually 

 irregularly. The flesh of varieties 

 with no anthocyanin varies from 

 white or cream to yellow. Unidenti- 

 fied carotenoids are among the 

 pigments. Purple-fleshed varieties 

 are favored over white in the 

 Philippine Islands because of the 

 agreeable color that they lend to 

 desserts. Nevertheless, anthocyan- 

 ins have no nutritional value, and 

 in the opinion of many persons, 

 distract from the appearance of 

 the cooked tuber. 



Both smooth- and coarse-tex- 

 tured varieties are found. Coarse 

 tuber texture, which appears curd- 

 like, is attributable to bundles of 

 storage tissue in a somewhat clear 

 matrix. 



When a tuber is cut, the flesh 

 exudes mucilaginous substances, 

 now known to be glycoproteins, 

 which can discolor the surface, 

 slowly or rapidly, upon oxidation. 

 Exposed to air, cuts are healed by 

 the formation of layers of dead, dry 

 cells. There is no evidence of suber- 

 in deposition in this species, but 

 dried cells effectively reduce mois- 

 ture loss and fungal penetration. 



