PATHOLOGIC CONDITIONS IN AMBROSIA TRIFIDA 
43 
bundles. This area is filled with a mass of parenchyma in which there are 
a few fibers but no vessels. There is a slight proliferation of the parenchyma 
into the gall cavity similar to that which has been described from the insect 
gall. There are no fungus sporanges immediately adjacent to this prolifera- 
tion, but they do occur close to what was formerly the cambium. Opposite 
the parts occupied by the fungus the bark is very irregular and thickened, 
as can be seen by comparing the two sides of the figure. 
We have well illustrated in this gall the effects of the two stimuli, fungus 
and insect, working together. Of the two, the stimulus from the fungus 
seems to be the stronger. In portions of the stem where the fungus was 
present very early in the growth of the bundles, all the structural character- 
istics are present which are commonly found in galls formed by the fungus 
alone. On the side of the stem where the fungus was not present in the 
early growth the structure is the same as in the insect gall. As the fungus 
spreads in a peripheral direction, however, those bundles which have already 
responded to the stimulus from the insect in their early growth immediately 
assume the structure of the fungus gall in the tissue formed after invasion. 
When the two stimuli act together on young tissue it seems to be true that 
only that from the fungus is capable of causing change in structure. When 
the fungus invades tissue already under the influence of the stimulus from 
the insect, the influence of the latter is destroyed or neutralized and the 
tissue formed subsequently is that of the fungus gall. 
It is unfortunate in this case that we have to do with an insect gall former 
which is incapable of causing violent disturbances in the host plant. It 
would be interesting to know what would be the result on the tissues of the 
host plant if two organisms, a fungus and an insect both capable of produc- 
ing profound tissue modifications, should exert their influence at the same 
time. 
Traumatic Wood of Ambrosia Trifida 
It is beyond the scope of this article to enter into a description of the 
various types of traumatic wood which result from the different kinds of 
wounds such as has been given by deVries (2) and Maule (4), because only 
general features of the traumatic tissue are of interest in this connection. 
It might be well to mention, however, that the types of wound wood ex- 
amined are quite similar in a general way to types described by these authors. 
Wood resulting from two sorts of wounds was examined. One of the 
wounded stems was collected along a roadside where a vehicle or some other 
moving object had torn away one side of the stem which had later started 
to heal over. The other was of a stem which had been broken over and 
badly twisted. The first of these stems was examined from cross and tan- 
gential sections, the second from tangential sections only. The cross 
section of the first shows a solid mass of wood in the wounded area, 8 mm. 
thick at the widest point, which had lost all appearance of having come from 
