3 88] ENGELMANN—OAKS OF THE UNITED STATES. 
17 
the former, while among the Black-oaks I find six forms, enumer¬ 
ated below, which I consider as real hybrids ; of them only a few, 
usually only single individuals, have become known? their exist¬ 
ence cannot well, without straining facts, be considered due to 
innate variability in a supposed parent. Four of these hybrids 
have been found in the Mississippi Valley and two in South Caro¬ 
lina. When more carefully looked for, undoubtedly more will be 
discovered. The four western hybrids all claim j|>. imbficaria 
as one of the parents, while the two southeastern ones seem to 
point to cinerea; both are species with entire leaves, and the 
mixture of such with lobe-leaved forms would of course be more 
readily discovered, from the intermediate leaf-form of the illegiti¬ 
mate offspring; but it remains for further close examination to 
ascertain whether lobe-leaved species do not hybridize among 
themselves, or entire leaved-forms also mix together, producing 
offspring of less strikingly marked characteristics. 
That we have to look for one of the parents of all our hybrids 
to only two species, seems to correspond well with other observa¬ 
tions, all pointing to the fact that some species of a genus are 
more prone to hybridize than others. This is true of Verbenas , 
hybrids of which abound in this neighborhood in numerous forms 
as well as in a great many individuals ; of most of them V. stricta 
appears to be one of the parents, perhaps because one of the most 
common species, or from some innate quality which makes it 
mix more readily with others ; perhaps from a peculiar structure 
of the flower which may promote insect agency. Our hybrid 
Verbenas differ from the hybrid oaks in bearing scarcely any fer¬ 
tile seeds, while at the same time they are so common that evi¬ 
dently they are readily produced anew. 
Our oak-hybrids are the following : 
Probable Parents. Name under which described. 
coccinea, imbricaria , - - - <^. Leana. 
rubra , “ - 
^ nigra , “ - - - - Jg. tridentata. 
Catesbcei , cinerea, - - - sinuata . 
falcata , ** - - - . - ■ 
1. c^. imbricario-coccinea was first described and figured by Nuttall, 
about thirty years ago, under the name of Jfh Leana , Nutt. Sylv. Contin. 
1, tab. 5 bis; DC. 1. c. 62. The original type was discovered by Mr. T. G. 
W 
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