ences that indicate separate species (3). In spite of Ayensu's 

 excellent distinction, many intermediate forms of the two species 

 occur, and these add to the confusion. Such forms are probably 

 hybrids or hybrid derivatives. However, the occurrence of 

 hybridization has not been rigorously demonstrated. 



The existence of wide variation among the varieties of D. 

 rotundata, and to a lesser extent, of D. cayenenesis, and the occur- 

 rence of other species in the same section cause classification 

 difficulties. Excellent keys in English have been developed by 

 Miege {21), Waitt {29), and Lawton and Lawton {16). However, 

 the first recognizes only one species, and the second and third, 

 two species. Among the key characters common to both species 

 are simple leaves, presence of spines, annual stems and tubers, 

 and absence of aerial bulbils. The males, when in flower, can be 

 identified from the flowers as well. 



Morphology 



D. rotundata and D. cayenensis are vigorous vines that climb 

 by twining to the right (fig. 2). Their foliage may be entirely 

 glabrous, and it may have a purplish waxy bloom. Spines are very 

 common, especially on the lower and larger stems. The stems 

 may be somewhat striated vertically, and the diameter varies 

 among varieties. The length of the vine varies, but heights of 10 

 meters are easily accomplished. 



The leaves are extremely varied (fig. 3). Their length varies 

 from 4 to 20 centimeters. Shapes are ovate, cordate, or almost 

 orbicular. The form of the leaf is also influenced by the degree of 

 folding, reflexing, cupping, undulation of the margins, and the 

 position on the plant. In addition, leaves vary in number of veins, 

 in presence and size of lobes, in the size of the sinus between 

 lobes, in rugosity, and in intensity of color. These traits, or 

 combinations of traits, are useful in identifying varieties. 



Leaf veins and stems contain variable amounts of antho- 

 cyanin, but in contrast to D. alata, the two African species are 



Table 1. — Most common differences between D. rotundata and 



D. cayenensis 



Trait 



Growing season 



Climatic preference 



Leaf shape 



Number of harvests 



Possible time of harvest . 



Tuber color 



D. rotundata 



7 to 8 months 



Intermediate rainfall 



Narrowly ovate 



2 



Limited, late summer 



to winter. 

 White 



D. cayenensis 



10 to 12 months. 

 High rainfall. 

 Broadly ovate. 

 1. 



Almost year round. 

 Yellow. 



5 



