graves: breeding trees for disease resistance 
97 
early spring. This, to me, is additional evidence of its hybrid nature. 
What I mean to say is that hybrids are often—not necessarily always — 
of a hardier nature and may also show greater vegetative vigor. 
There are therefore four indications of the hybrid nature of the London 
plane, Platanus acerifolia . 
1 . The varied character of the seedlings and the fact that some of 
them resemble the oriental, others the occidental parent, and others 
the hybrid itself. 
2 . The remarkable adaptability of the tree. 
3. The fact that it has never been found in the wild state. 
4. The fact that in the tree itself some characters are like those of one 
parent and others like the other parent. 
This evidence is, of course, only circumstantial. We have no direct 
proof of the crossing of the oriental and the occidental species. Per¬ 
sonally, I am inclined to the belief, on the basis of the evidence, that it is 
a hybrid. The only other possible explanation is that it is a variant of 
one of the above-named species. However, the fact remains that it is a 
different tree from the oriental plane and the sycamore. 
Unfortunately for my thesis a disease of the London plane was re¬ 
ported in 1935 by Jackson (11th Nat. Shade Tree Conf. 77-79) as being 
prevalent in the Philadelphia area. The causative agent is a species of 
Ceratostomella. According to the report, there may be predisposing 
causes to the disease, such as injury from sidewalk and curb improve¬ 
ments. We in New York City, where thousands of these London planes 
are planted, have not noticed any such disease, or at least if such has 
been reported I am not aware of it. However, until this cloud dis¬ 
appears (if it does) it would be wiser to “go slow” in planting London 
planes. 
As far as I can find out, no work has been done—that is, in a syste¬ 
matic way—in breeding shade trees. At any rate, no published results 
of such work are available, although I believe that Dr. E. J. Schreiner 
of the Northeastern Forest Experiment Station is including this objec¬ 
tive in some of his crossing work. 
That such work should be practicable is shown by our own experience 
at the Brooklyn Botanic Garden in crossing various species and hybrids 
of chestnut trees. This we are doing primarily to develop resistance to 
the blight. We have found that this crossing work is easily done if one 
knows how and if the various operations are carried on at the right time. 
