166 BACTERIA. FUNGI. 



the bark in the form of small scarlet shields. Again, we have the yellow Poly- 

 porus stilfureus with its immense yolk-coloured, bracket-like fructifications, which 

 in the space of a week grow out from the trunks of larches, although the outward 

 appearance of the host gives no indication of its being completely occupied 

 internally by a mycelium. Polyporus betwlinus and P. fomentarius likewise 

 grow to a considerable size, and in both cases it is specially deserving of notice 

 that the colour and structure of the surface of the fructification is surprisingly 

 like the bark of the trees upon which they are respectively parasitic; that is to say, 

 the fructification of Polyporus hetulinus strongly resembles the whitish bark of 

 the birch, and that of Polyporus fomentarius, parasitic on old beech-trees, exhibits 

 the same pale gray as does the trunk of a beech. 



Mildews form in some respects a contrast to these parasites whose hyphffii pene- 

 trate into the interior of their hosts. They attack tender green leaves, stems, and 

 young fruits, and accomplish their entire development upon the epidermal cells of 

 the hosts. At first sight the parts assailed appear to be strewn with flour or dust 

 from the road. But on closer inspection a delicate weft is to be distinguished, 

 composed of filaments ramifying extensively upon the green substratum, intersect- 

 ing one another, uniting to form reticula, and in parts a regular felt- work covered 

 at certain spots with the small dark spheres of the sporocarps. Individual hyphse 

 of this weft adhere closely to the epidermal cells of the host, dissolve the outer 

 walls of these cells at the points of contact, so as to make little apertures, and then 

 develop processes which grow into the interior of the epidermal cells in question, 

 assume a club-like form, and exhaust the cell-contents. The mycelia of mildews 

 do not penetrate into the host beyond the epidermal cells. Fig. 32 - shows a piece 

 of a leaf of Acanthus mollis attacked by mildew, with hyphal suckers penetrating 

 into the epidermal cells of the leaf. One of the best-known mildew fungi is 

 the Vine-mildew {Erysiphe Tuckeri), which weaves itself over the epidermis of 

 still green and unripe grapes, and has frequently manifested itself through the 

 districts where the vine is cultivated in southern and central Europe in the form of 

 a ravaging disease. 



The protuberances sent by the hyphse, in the form of clavate swellings, or more 

 rarely winding tubes, into the cells of the host-plants, correspond to the absorption- 

 cells of land plants, and the conditions under which suction takes place are 

 essentially analogous in the two cases. The absorption-cells on the roots of land 

 plants do not take in all the substances in their nutrient substratum, and similarly 

 the hyphse only appropriate by means of their organs of suction a portion of the 

 contents of the cells invaded. They begin by dissolving and breaking up for this 

 purpose the substances in the infested cells of the host. What compounds they 

 then select from among the p:-oducts of decomposition, and what they leave behind, 

 cannot certainly be specified in detail. It is believed that, m many cases, tannin 

 is appropriated first of all by parasites. The wood of a healthy oak, for instance, 

 has a characteristic smell due to the abundance of tannin it contains, whereas this 

 odour is not emitted by wood attacked by the mycelia of fungi, and this decayed 



