TROPICAL YAMS AND THEIR POTENTIAL, PART 3 



19 



have produced more tuber yield 

 than comparably sized pieces of 

 large underground tubers. 



Small, whole, underground tu- 

 bers of D. alata are sometimes used 

 for seed. Although this practice 

 might appear similar to planting 

 an aerial tuber, small underground 

 tubers often give poor results be- 

 cause their smallness may be the 

 result of abnormal growth. Studies 

 in Puerto Rico have shown that a 

 large proportion of small tubers 

 come from virus-infected plants, 

 and plants grown from them are 

 usually infected with virus. 



The vast majority of yams are 

 planted from pieces of mature 

 tubers. Yams for seed should be 

 selected from the largest and 

 healthiest and stored at moderate 

 to cool temperatures and moderate 

 to low humidity. Seed pieces cut 

 from tubers should weigh 100 to 

 500 grams. Planting is usually 

 timed to coincide with the begin- 

 ning of the rainy season. When 

 sprouting begins in a few tubers, 

 tubers should be sectioned and 

 planted a few days later. When 

 cutting tubers for seed pieces, rot- 

 ten portions should be discarded. 

 Cutting tubers weeks in advance 

 or permitting sprouting before 

 cutting are common practices but 

 are not recommended. The cut sur- 

 faces should be dusted with wood 

 ashes or a dry fungicide mixture 

 and allowed to dry a little before 

 planting. This procedure prevents 

 rot in the soil, preserving the nu- 

 trients stored in the tuber piece for 

 the growing plant. 



Seed pieces are usually made by 

 cutting the yam crosswise. The 

 piece including the plant crown 

 has preformed buds and germi- 

 nates readily. The tender growing 

 tip of the tuber often does not 

 germinate very well. (However, 

 there is disagreement on this 

 point.) The midsections usually 

 germinate well after new buds 

 have developed. To avoid the prob- 

 lems of middle and tip sections, the 

 crown ends of tubers to be used for 

 food can be cut off and saved for 

 seed. These pieces, which are 

 tougher and more fibrous than the 

 remainder of the tuber, are likely 

 to have low value as food anyway. 

 Clarification of the relative ease 

 of germination of the various sec- 

 tions as related to tuber age and 

 cutting time is desirable. 



The germination of seed pieces 

 can be stimulated to some extent. 

 High temperatures (40°-60° C) 

 for 1 hour or more advance germi- 

 nation 1 or 2 weeks. Planting 

 stimulates germination, probably 

 by furnishing warm and humid 

 conditions. Cool, dry storage in- 

 creases the dormant period. There 

 are limits to these techniques, but 

 they have not been investigated 

 systematically. The biological 

 clock mechanism of the variety is 

 probably the most important 

 factor determining time of germi- 

 nation. 



Chemical treatments have been 

 useful in stimulating germina- 

 tion. An 8 percent solution of ethyl- 

 ene chlorohydrin (2-chloroetha- 

 nol) used as a dip 4 to 6 weeks 



