Reproduction 375 



water (transpiration) may be excessive and death by dry- 

 ing-out is then a chief cause of failure. In all cases the 

 reduction of the transpiratory surface to a minimum is 

 required, and it is essential that the conditions of the cut- 

 ting bench shall be most favorable with respect to moisture, 

 drainage, light, and temperature. The exposed cut sur- 

 faces are also more subject to the attacks of hemi-parasitic 

 or damping-off fungi ; therefore, the cutting bench re- 

 quires the same careful attention as the seed-bed. In 

 many cases a thorough knowledge of the growth habits of 

 the plants and the best skill of the gardener will be required 

 to determine the conditions needed. It may be neces- 

 sary by special means to induce root development in the 

 portion to be used for a cutting before the branch is sepa- 

 rated from the stalk, as by attaching a pot with moist soil 

 or moss. Special cases, however, are too numerous to 

 receive consideration. 



224. Vegetative reproduction and running out. - - Some 

 observers have held that vegetative reproduction repeated 

 through numerous generations results in deterioration; 

 but many or all of the cases cited in substantiation of this 

 view are, in the opinions of others, wholly invalid. Never- 

 theless, in one sense varieties may " run out." Thus bud 

 variation may be so great that in time the original form 

 may be entirely lost; this is " varying out." 



It would seem that the yam (Dioscorea saliva) has been 

 vegetatively propagated in China for two thousand years, 

 and there is no evidence that it is decadent. The sweet- 

 potato has apparently long lost the power of seed produc- 

 tion, and we cannot assume that it has lost in vegetative 

 vigor. The fig and the date have not been commonly 



