THE BLACK POPLARS. 1 5 



areas occupied by two distinct species, are more likely to arise 

 in nurseries or gardens where several species are cultivated 

 together. The fine elm,^ which is universally planted in 

 Holland and Belgium, is not a natural species or variety 

 peculiar to those countries, but a hybrid, as is shown by the 

 nature of its seedlings. It is invariably propagated by layers, 

 and originated probably as a single tree about 400 years ago. 



Sports in trees arise in two ways, either as abnormal and 

 often solitary seedlings in the seed-bed, or as a single peculiar 

 branch on an otherwise normal individual. Sports, like hybrids, 

 are usually propagated by grafting, layers, cuttings, etc. Both 

 sports and hybrids may, through the readiness with which they 

 can be multiplied, exceed natural species or varieties in numbers 

 of individuals. Sports are generally selected for the curiosity or 

 beauty of their appearance; whilst hybrids, especially first crosses, 

 are mainly valued for the vigour of their growth. The origin of 

 many cultivated trees can only be elucidated by a study of. an 

 historical character, supplemented by experiments, such as 

 sowing large numbers of seeds to ascertain the characters of the 

 resulting offspring. No genus shows more clearly than Populus 

 the distinctions between natural species or varieties, sports, and 

 hybrids. 



The two wild species ^ of black poplar with which we are 

 concerned in this article occur, one in Europe, the other in 

 eastern North America. The European black poplar is dis- 

 tinguished from the American species by the absence of two 

 characters in the leaves, viz. cilia on the margin and glands on 

 the base of the blade in front. These are invariably present in 

 the American species. In the flowers of the latter the stamens 

 are 40 to 60, and the stigmas and placentae, 3 or 4 ; in those of 

 the former there occur 12 to 25 stamens, and only 2 stigmas and 

 2 placentae. The shape of the leaf is markedly different ; in the 

 American species it is deltoid-ovate, as broad as long, contracting 

 abruptly into a cuspidate apex, the base being shallow-cordate 

 or truncate. In the European black poplar the leaf is longer 

 than broad, being drawn out at the apex into a long acuminate 

 point, while the base in well-developed leaves is wedge-shaped. 



' Uhmis lalifolia, Poederle. For its history, see Trees of Great Britain, 

 vii., pp. 1869, 1870. 



'^ The only other species in cultivation in England, not mentioned in this 

 article, is Populus Fremontii, Watson, which is still a rare tree in botanic 

 gardens. It has leaves ciliate in margin, without basal glands. 



