K.—BOTANY 181 
their hosts, many fungi which invade the host tissues through wounds and 
which can live also in a purely saprophytic manner generally shew no pro- 
nounced degree of specialisation to particular hosts. Stereum purpureum 
from a birch stump, for instance, can attack a wide range of living trees such 
as plum, apple and laburnum, and Polyporus squamosus can invade many 
kinds of broad-leaved trees. Apart from Stereum purpureum and a few 
other species there is little precise information yet available concerning 
the initiation of infection by fungi which obtain entry into the host through 
exposures of the wood. We do not know exactly, for example, how such 
common fungi as Polyporus squamosus and P. betulinus infect living trees. 
With Stereum purpureum Moore and I ** have shewn that prior to infection 
the spores are usually sucked a considerable distance into the vessels, 
where their germ tubes are in less danger of desiccation than on the exposed 
surface. In this connection it is of interest that the spores of most of 
these wound parasites are much narrower than the diameter of the vessels 
in the wood of their hosts. Experiments with many such fungi indicate 
that under certain conditions the spores are readily drawn into the vessels, 
and this is probably an important factor in the initiation of attack by fungi 
of this class. Again, S. purpureum much more readily invades fresh wounds 
in the wood than those which have remained under the influence of the 
host’s response to wounding and the action of other micro-organisms.*® 
This fungus can also infect new wounds with greater facility at some 
periods of the year than at others, probably in correlation with the varying 
metabolic condition of the tree. Whether such factors operate in connec- 
tion with other fungi which infect their hosts through exposures of the 
wood is not known. In large trees the heart wood differs greatly from 
the sap wood, especially in containing no living cells. It is known in a 
general way that some fungi, e.g. Polyporus squamosus, attack the heart 
wood more vigorously than the sap wood, whereas others spread chiefly in 
the sap wood, but further information is required concerning the influence 
of these two classes of woody tissues on the initiation of invasion by fungi. 
There is also a wide field for further research on the early stages of infec- 
tion of plant tissues by other classes of fungi which live sometimes as 
saprophytes and sometimes as parasites. Thanks to Marshall Ward,°’ 
Blackman and Welsford,*8 Brown *® and others we now have an dinivest 
complete picture of the initiation of parasitism by Botrytis cinerea and 
allied fungi in which the secretion of toxic enzymic substances plays the 
principal réle. It is perhaps in the initiation of attack that the most 
interesting features of fungal parasitism are shewn. ‘This is exemplified 
in Green’s*° recent investigations on the rots of oranges caused by 
Penicillium digitatum and P. italicum, mould fungi which are familiar to 
everyone. In addition to infection through wounds these fungi can infect 
perfectly sound fruits under certain conditions, although the outer yellow 
85 Pyoc. Cambridge Philosophical Soc. (Biolog. Sci.), 1, p. 56 (1923). 
86 Jour. Pomology and Hort. Sci., 5, p. 61 (1926). 
37 Ann. Bot., 2, p. 319 (1888). 
38 Ann. Bot., 80, p. 389 (1916). 
89 Ann. Bot., 29, p. 313 (1915). 
40 Jour. Pomology and Hort. Sci., 10, p. 184 (1932). 
