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 hyphse 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 hyphse 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 liable 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 hyphge. 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 hyphse 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 hyphse 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 hyphse of Polyporese, 

 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 Peronosporeae, prefer 

 to bury themselves in the passages between individual cells, i.e. in the so-called 

 intercellular spaces. The hyphse 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 excrescences, which are named 

 haustoria, the parasite sucks the substances required for its own nourishment from 

 the living substance of the penetrated cells. 



The hyphse of the above-mentioned parasitic fungi have the peculiarity that in 



