508 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



by means of a lens, forming minute black spots on the affected areas 

 on the upper surface of the leaf (fig. 89, b). I have only once seen these 

 microsclerotia just within the lower epidermis, and in that case the lower 

 side of the leaf had curved over and was directed upwards. From the 

 microsclerotia later arise the fertile hyphae, or conidiophores, in little 

 tufts of black or dark brown, erect or somewhat spreading, branches. The 

 conidiophores are unbranched and sparingly septate, and bear at their tips 

 several conidia. As the hyphae which form these tufts are somewhat 

 rigid, they may be seen standing up from the surface of the leaf, by the aid 

 of a lens (fig. 91, d). Conidiophores, occurring singly, are occasionally 

 found emerging from the lower surface. The conidia (fig. 91, e, f) are dark 

 in colour and very variable in form, size, and number of cells, sometimes 

 being unicellular, but more frequently consisting of two or three cells. 

 The conidia are capable of reproducing the disease spots upon other apple 

 leaves. 



The Origin of the Parasitism. — The fungus is, as has already been 

 stated, a very common saprophyte on dead vegetable matter, and it can, 

 furthermore, be grown with ease on an ordinary culture medium such 

 as beef- tea jelly. It is interesting to note that several of the allied 

 species of the genus are normally active parasites. For example, 

 Cladosporhtm fuivum is a species too well known upon the cultivated 

 tomato, while C. elegans is the cause of a " scab " on oranges, and so on. 

 How is it that this species is also capable of becoming a parasite? 

 Considerable light has recently been thrown upon this interesting question 

 by various workers among the fungi. In 1894 Miyoshi * showed that 

 the hyphae of fungi are attracted through small openings in a membrane, 

 such as the stomata in the epidermis, by the presence of sugar on the 

 other side. Massee t has further shown that it is possible so to train 

 a saprophytic fungus, by attracting it to grow in and fruit upon a plant, 

 that after a certain number of generations it acquires the power of 

 spontaneously infecting the plant upon which it has been enticed to 

 grow. Many fungi are capable of adapting themselves to their environ- 

 ment in a very marked manner, for certain forms quite indistinguishable 

 from one another in appearance are able to infect one plant only of a 

 number of allied forms or species, while other forms of the same fungus 

 are restricted to other of the allied species of host plant. These " biologic 

 forms " have been carefully investigated, particularly by the late Professor 

 Marshall Ward and by E. S. Salmon, and it has been shown that there 

 are often certain hosts that can be infected by several of the " biologic 

 forms " of a parasite, though these forms are otherwise restricted to one 

 or two hosts, and do not invade each other's territory ; but, after 

 growing upon the common host, they are able to infect other hosts 

 indiscriminately. These facts go to show that the food relations of a 

 fungus are often not fixed, but that they are able to adapt themselves to 

 altered circumstances, and even to acquire what appear to be new powers, 

 such as the power of attacking living plants when they have hitherto been 

 growing only upon dead matter. The species Cladosporium herbarum is 

 a case in point. A possible explanation of the fact that this species has 



* Bot. Zeit. part i. (1894). 

 t I'hil. Trans. Roy. Sue. Ser. B, 197, p. 7 (1904). 



