630 VITAL ACTIONS OF THE LOWER FUNGI. 



various. At times they gradually destroy the whole of 

 the surrounding tissue, so that a mass of fungi, con- 

 sisting of mycelium and spores, occupies the place of the 

 original vegetable substances (ustilagineae, exoascus) ; 

 or alterations of a more local character arise, for ex- 

 ample, marked growth of the parenchyma, which leads 

 to all sorts of swellings and alterations in form (chytri- 

 diaceae, cystopus, &c.) ; or, finally, local alterations are 

 less noticeable, but instead a gradual degeneration of the 

 surrounding tissue occurs, which shows itself in dis- 

 colouration and browning of the cells attacked. In by 

 far the greatest number of cases the infection leads to 

 death of the plant, or to death and degeneration of the 

 fruit. 



individual Among the large number of mould fungi there are rela- 



ofthe pant n ^ ve ^J ^ ew species which possess this pathogenic power, 

 and we must assume that these species are provided with 

 particularly powerful means of penetrating the cell mem- 

 brane. Further, each of the pathogenic forms can only 

 attack one or a few species of plants, and among plants 

 which are apparently similar there are often only a few 

 individuals which are specially predisposed to the action 

 of the parasitic fungus. Minute differences in the 

 structure of the epidermis and of the cell walls, slight 

 variations in the chemical composition of the cell juice, 

 greater or less energy of growth and of tissue change, 

 are the conditions to which must be referred the con- 

 nection between certain parasitic fungi and certain 

 plants, the immunity of other plants, and, in short, the 

 individual predisposition to the infective diseases. The 

 epidermis of the plants seems to oppose an especially 

 great resistance against the fungi, and hence infection 

 can often only occur while the parts attacked are in a 

 young condition and still possess a delicate skin. Thus 

 the ustilaginese and peronospora inf. only attack young 

 plants or young seed ; hypoderma macrosporon only 

 penetrates into young pine cones. An injury of the 

 epidermis is often requisite to enable the mycelium 

 threads to penetrate it ; species of fumago only develop 

 on parts which are attacked by aphis ; in the case of 



