TROPICAL YAMS AND THEIR POTENTIAL, PART 3 



5 



PN-4437 



Figure 1. — Female flowers of D. alata. 



cies can usually be excluded read- 

 ily. D, alata is a glabrous vine that 

 climbs by twining to the right. The 

 length of the vine varies from 2 to 

 30 meters or more in primitive 

 forms. 



The fleshy stems are character- 

 ized by wings, typically mem- 

 branous and often ruffled. In some 

 varieties the wings are reduced or 

 almost absent, but the stems may 

 be quite thorny, in which cases 

 the thorns occur in straight lines 

 representing the wing; the habit 

 is associated with two distinct 

 tuber types and with a distinct 

 leaf shape. In rather primitive 

 varieties with well-developed 

 wings, the wings of the thickened 

 basal stem are often modified into 

 thick, blunt thorns quite different 



from the thorns of the Feo type. 

 Wings are believed to help the 

 stem grasp smooth objects in twin- 

 ing. 



The leaves are often quite large, 

 although those of the better vari- 

 eties may be smaller. They are 

 commonly opposite on the stem, 

 but in a few varieties and on the 

 young vigorous stem of most vari- 

 eties, they are alternate. The leaves 

 are glabrous, with a slight bluish 

 bloom in a few varieties. Many 

 varietal differences can be dis- 

 tinguished (fig. 2). The shape of 

 the leaf is determined principally 

 by the width of the sinus between 

 the leaf lobes. Varieties differ in 

 the folding and undulation of 

 leaves, in the thickness of the 

 lamina, in green color intensity, 

 and in anthocyanin coloration. The 

 wings of the petiole of the leaf are 

 usually enlarged at the stem, pro- 

 ducing a characteristic appear- 

 ance. Stipules are rarely present. 



The flowers of D. alata are pro- 

 duced during the last third of the 

 growing season, when tubers are 

 forming. Not all varieties flower, 

 and some flower sporadically, per- 

 haps in response to seasonal vari- 

 ations. It is unknown whether 

 flowering can be induced by chang- 

 ing environment. 



Male and female flowers are 

 borne on different plants. The 

 male flowers are small (1-2 milli- 

 meters in diameter) and are pro- 

 duced in crowded panicles that 

 originate in the axils of the leaves 

 and the tips of the branches. The 

 sessile flowers range from green 



