154 BRITISH PLANTS 



e.g., the weeping varieties of ash and willow, and the 

 variegated varieties of Aucuha, privet, maple, geranium, 

 etc. When the sporting character arises in the seed, it is 

 generally transmitted by seed ; when it does not, vege- 

 tative modes of reproduction are alone capable of pre- 

 serving it. We have already drawn attention to the fact 

 (p. 149) that the best varieties of cultivated fruits are 

 multiplied by vegetative means, and that if, this is done 

 indefinitely the seeds tend to become obsolete. It follows 

 from this that the vegetative mode of multiplying indi- 

 viduals, if very successful, is Kkely to outrun, and finally 

 to supersede, the sexual mode by seed. The common elm, 

 for instance, is so easily propagated by cuttings and root- 

 suckers that fertile seed is rare in this coimtry. 



Nevertheless, for the great majority of plants, repro- 

 duction by seed is either the only method, or it is the 

 method which must occasionally intervene to preserve 

 the continuity of the race. A long course of vege- 

 tative reproduction tends, in many cases, to exhaust the 

 stock, and many domestic races so reproduced are only 

 saved from extinction by careful and selective cultivation. 

 Sometimes even then constitutional delicacy becomes at 

 last so pronounced that new strains are started by crossing 

 with wild stocks and raising a more vigorous race from seed. 



In those plants which reproduce vegetatively as well as 

 by seed, one mode usually predominates over the other. 

 If conditions are favourable for vegetative development, 

 the vegetative mode prevails ; if they are unfavourable, 

 seed. Thus, the lesser celandine {Ranunculus Ficaria) 

 multiplies equally well by tubers and seeds. In damp 

 shady places tubers are formed in abiuidance, and there 

 are few flowers. In dry sunny spots few tubers are 

 formed, but the plants flower freely, and seed is produced 

 in abundance. The production of seed bears an inverse 

 proportion to the number of tubers formed. 



Modes of Vegetative Reproduction. 



1. By Underground Stems or Rhizomes (p. HO). — Any 

 portion of the rhizome which bears a bud, and contains a 

 sufficient store of reserve-food, is capable of independent 

 existence — e.g., couch-grass (Fig. 28), dog's-mercury, iris, 

 wood-sorrel, mint (Fig. 29), etc. 



