12 BULLETIN 360, U. S. DEPARTMENT OF AGRICULTURE. 
were killed by the parasite. The other trees in the figure show 
various stages of suppression and an abnormal thinness of foliage. 
The tree on the extreme right shows midway on its trunk a typical 
mistletoe trunk burl. 
It is often disputed that mistletoe is a cause of spiketop or that 
it is totally unknown for some species. The first and heaviest seat of 
infection in nearly all trees of economic importance is in the lower 
part of the crown (figs. 6 and 8). This is not necessarily a result of 
the seeds of the parasite falling first on the lower branches, but is 
rather the result of the fact that the main shoot continues for a time 
to grow in height, and the crown may attain its normal height be- 
fore the effects of the parasite become dominant. The mistletoe 
spreads upward from the lowermost branches, with the result that 
the more recently formed branches are continually being infected. 
That these infections may not cause a brooming of the branches in 
the beginning is abundantly shown by the entire absence of any 
brooming on young infected branches of several host species. This, 
however, is only the first stage in the hypertrophy of the branch. 
After the lapse of several years, typical brooms are formed. With 
the increasing hypertrophy of the lower portion of the crown, food 
materials are more and more appropriated at this point. The result 
is a drain on the resources of the entire tree to support the brooms. 
Materials traveling upward from the roots are likewise utilized by 
the broomed branches, with the result that the upper portion of the 
crown starves and in cases of severe infection finally dies (figs. 5, 6, 
7, and 8). Spiketop is an almost universal condition in heavily 
infected larch. The tendency to form spiketop in this species, how- 
ever, is greatly augmented by the brittleness of its branches. Douglas 
fir probably comes next in order of frequency of dead tops resulting 
from the growth of mistletoes. The condition is common for yellow 
pine in all regions where observations have been made by the writer 
and is reported to be of frequent occurrence by correspondents in 
Utah and Wyoming. Lowland and mountain hemlocks, when heavily 
infected, quite commonly exhibit dead tops. An unusual case of 
heavy infection of the former species was studied in the St. Joe 
National Forest. Practically every tree in the entire stand was dead 
in the top (fig. 9). Lodgepole pine is less affected in this manner 
than any other conifer so far studied by the writer except spruce 
and fir. The last-named species are so seldom infected, however, 
that they would not enter into the discussion. 
There can be little doubt that spiketop is very often the result 
of heavy mistletoe infection, but varies in degree for the several 
hosts. This condition is of importance, since the proportion of 
snags in the stand is thereby increased, which may promote injury 
by fungi and insects; it also increases danger from lightning fires. 
