164 BACTERIA. FUNGI. 
surface of the plant serving as host has become broken—wounds occasioned by 
animals, violent wind, hailstones, or the weight of superincumbent snow—and use 
these as means of ingress. Yet others adopt the shortest route by breaking through 
the wall and so effecting an entrance for themselves. The tips of the hyphi and 
also of the outgrowths developed by them have the power of decomposing and 
destroying the membrane of cells in the living plant serving as their host. At the 
spots to which they apply themselves, little gaps are shortly produced in the cell- 
membranes, and through them the hyphz penetrate, either in their entirety 
or by means of special processes, into the interior of the cells attacked. In 
this operation it does not matter whether the hypha concerned has just emerged 
from a germinating spore or is a ramification of a mycelium several years old, 
which has been quiescent for a time and then begun to germinate again vigorously; 
the power of perforating cell-walls is a property possessed by the one as much as 
the other. 
The aspect of the host’s epidermal cells at the places where the hypha comes 
into contact with its victim is, on the other hand, not quite such a matter of 
indifference. For plants lable to become hosts are not without contrivances for 
protecting themselves against intruders. Thus their epidermal cells have their 
external walls greatly thickened and invested with cuticle. Although the main 
object of this is merely to afford protection against excessive transpiration and 
desiccation of cells filled with sap, a thickening of the kind constitutes also a coat 
of armour which is not liable to be broken through by every hypha. Still greater 
security is afforded by a double or triple layer of thick-walled cells destitute of 
sap, such as a solid corky bark. Coats of this kind are not penetrated even by the 
most vigorous hyphz. In order to gain admittance notwithstanding, many force 
their conical tips into the fissures and crannies of the bark, push the peeling scales 
apart or even burst them, and so succeed ultimately in reaching parts which are 
susceptible of being pierced and allow the hyph to conduct their mining operations 
with effect. In the majority of cases the parasite is not content with perforating 
and exhausting the superficial cells alone of the host; its hyphz grow faster as 
they penetrate deeper, a process generally accomplished irrespective of the number 
or direction of the partition walls in their way. Thus the hyphe of Polypore, 
which are parasitic in the wood of living trees, penetrate whole series of cells, now 
growing through a bordered-pit, now piercing the uniformly thickened part of the 
wall of a wood-cell (see fig. 32°). Others, as, for instance, the Peronosporex, prefer 
to bury themselves in the passages between individual cells, .e. in the so-called 
intercellular spaces. The hyphe imbedded in this way then develop lateral out- 
growths which perforate the walls of the cells adjoining the intercellular space, and 
upon entering the interior of the cells swell up to the shape of a club (see fig. 32 *). 
By means of these clavate or almost spherical exerescences, which are named 
haustoria, the parasite sucks the substances required for its own nourishment from 
the living substance of the penetrated cells. 
The hyphe of the above-mentioned parasitic fungi have the peculiarity that in 
