Reproduction 373 



and take root a few inches or more from the parent plant. 

 Advantage may be taken of all such nonsexual methods 

 in practical propagation. Moreover, in all such cases the 

 vegetative method enables the producer to be sure of the 

 variety or form, the propagation of which he is continuing, 

 since there is then, at least, no chance of mixing by hy- 

 bridization, or of change through segregation. As a rule, 

 plants that reproduce in a vegetative manner occupy the 

 land quickly. The method is, of course, of great service 

 when the plants are useful, but it may be most trying when 

 this habit is that possessed by a persistent weed. 



221. Thickened roots and tubers. Irish potatoes and 

 Madeira vines are types of plants propagated by tubers or 

 thickened stems, produced generally only by underground 

 buds. In the varieties of the potato commonly grown the 

 seed-ball is now seldom seen, so that there are varieties 

 the existence of which, in culture at least, is dependent 

 upon vegetative reproduction. Sweet potatoes, yams, 

 dahlias, and other familiar plants are propagated by 

 thickened roots. Many of these forms have in cultivation, 

 under ordinary conditions, lost the power of seed produc- 

 tion. The propagation of some edible and many floricul- 

 tural plants by bulbs and corms is so common among 

 liliaceous genera that the production of bulbs now repre- 

 sents a group of special industries. Holland is famous the 

 world over for this type of work, but doubtless the condi- 

 tions there afforded are practically duplicated in many 

 other places. 



222. Cuttings. A countless array of ornamental 

 herbaceous forms and some small and bush fruits are 

 regularly propagated by cuttings. In fact, carefully 



