been used successfully in Puerto Rico to protect the cut surface 

 from rot and are recommended here. Tubers should not be allowed 

 to develop significant numbers of sprouts before planting. Re- 

 moval of sprouts reduces vigor of future sprouts and although 

 necessary at times, can hardly be recommended. The best practice 

 is to cut tubers into seed pieces when sprouting just begins. 



D. rotundata may also be propagated from stem cuttings 

 (fig. 8). New stems are cut into sections with two leaves at one 

 node and a portion of the stem on either side of the node. The 

 cutting is sometimes treated with a hormone to induce rooting, 

 and the stem portion is buried in sand, gravel, or other rooting 

 medium. Intermittent mist spray is used to prevent moisture loss 

 until rooting occurs. Roots are formed from an undifferentiated 

 tissue, the primary nodal complex that forms at the axil of the 

 leaf. From this tissue the stem and the tuber emerge later. 



Cultivars of D, rotundata vary in their response to rooting 

 from cuttings. Some root readily and some are stubborn. Further- 

 more, the fact that roots develop does not mean that shoots or 

 tubers will form later. Depending on the cultivar, a variable 

 proportion of cuttings do not develop into entire plants within a 

 reasonable time. The technique, therefore, needs more intensive 

 study before it can be recommended. At the present time it is 

 useful chiefly as a method of multiplying a desired plant rapidly. 



Optimum planting distances depend on many factors. The 

 minimum distance between plants is 1 meter, but conventional 

 mounds are difficult to construct at such a small distance. Mounds 

 are usually about 2 meters apart. Ridges are spaced from 1 to 2 

 meters. Maximum size of tubers is obtained when plants are 2 

 meters apart. Closer spacing up to 1 meter results in decreased 

 tuber size but more yield per unit of area. In addition to these 

 considerations, the size of the planting piece can be smaller when 

 planting distances are reduced. 



After planting, sprouting proceeds at a pace relative to the 

 kind of materials used, the size of the seed piece, the planting 

 date, and the weather. Seed pieces need some moisture to germi- 

 nate, but excessive moisture can lead to rotting of the tuber 

 pieces. Appropriate moisture speeds germination but cannot 

 completely overcome dormancy if the tubers are planted too early. 

 On the other hand, sprouting occurs at the appropriate time of 

 the year whether or not tuber pieces are exposed to appropriate 

 conditions. The optimum time for planting is thus something 

 between extremes — not too early, but before sprouting occurs. 

 The time necessary for sprouting when tubers are planted just 

 before natural sprouting occurs varies from 2 to 10 weeks. 



Chemicals have been used to stimulate sprouting of tubers 

 of other yam species. However, these have not been tried with 



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