56 BOTANICAL GAZETTE [JANUARY 
nothing as to its origin, and synthetization and anatomic examinations 
are the only recourse. On the other hand, if the progeny exhibits 
alternate inheritance, its components may show unmistakably the 
nature of the original cross. 
The last-named method demonstrates beyond reasonable doubt 
that Bartram’s oak is a hybrid derivative of the willow oak and red 
oak, the progeny being probably characterized by alternative inheri- 
tance of some of the qualities, and fractionization of others, instead 
of being a unified hereditary strain. Some of the plantlets included 
in a progeny of 55 individuals were apparent re-constructions of the 
ancestral types as observed at the close of the first year, while others 
were variously intermediate. 
Rudkin’s oak, on the contrary, yields no evidence in cultures 
of its progeny on which a defensible conclusion as to its origin may 
be based. The individual variability of the trees included under 
this name is very great, and some of these approach the willow oak in 
leaf-form, and some in the character of the acorns. The study of 
a large number of seedlings from three trees showed the willow oak 
in leaf-form and some of the characters of the acorns. The study of 
over a hundred plantlets showed a wide and practically identical 
range of variation. Alternative inheritance could not be traced. 
The individuals known under the name of Q. Rudkini produce a 
large proportion of imperfect acorns, but beyond this no facts sug- 
gestive of hybrid origin can be found, except the anatomical resem- 
blances noted. 
A list of the natural plant hybrids of North America was prepared 
by Mr. Davip Georce for presentation at the International Hybrid 
Conference in New York in 1902, but was not published. In this 
list it was noted that 117 natural hybrids had been reported as 
occurring in the indigenous flora of North America, in addition to 
some instances among the ferns and mosses. The manuscript having 
been placed at my disposal by the director of the New York Botanical 
Garden, a revision of the list has been made, some of the original 
references being omitted and a few added. No attempt has been 
made to make it actually complete, the sole purpose being to suggest 
material for extended observations similar to those described in this 
paper. The reported hybrids are distributed among the natural 
families as follows: 
