154 BRITISH PLANTS 
e.g., the weeping varieties of ash and willow, and the 
variegated varieties of Aucuba, 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 likely 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 country. 
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 abundance, 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. 110).—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), ete. 
