There is yet a further point to notice here. It has been proved that certain substances formed in plant-cells, not necessarily nutritive, attract the hyphae of parasitic fungi or repel them, according to the kind and degree of concentration. So clear has this proof been made that it was possible in experiments conducted apart from a host plant, to make the hyphae on one side of an artificial membrane--_e.g._ collodion--penetrate it by placing one of these attractive (_chemotropic_) substances in suitable proportions on the other side. The hyphae dissolved holes in the membrane by means of enzymes and plunged into the attractive substance on the other side. The foregoing sketch gives us a glimpse into the causes at work in parasitism. Suppose a fungus on the outside of the epidermis of a young organ--say a leaf. It may be unable to penetrate into the plant, and finding no suitable food outside it dies: or it may be satisfied with the traces of organic matter on the epidermis and then lives the life of a saprophyte. Or it may be able to establish a hold-fast on the tender epidermal surface, but without entering the cells, and irritate the developing organ by contact stimulation, inducing slight abnormalities; if in its further, purely superficial growth such an epiphyte covers large areas of the leaf, and especially if the hyphae are dark coloured--_e.g._ _Dematium_ and other "Sooty Moulds"--injury may be done to the leaf owing to the shading action which deprives the chlorophyll below of its full supply of solar energy. Some epiphytes, however, are able to fix their hyphae to the epidermis by sending minute peg-like projections into the cuticle--_Trichosphaeria_, _Herpotrichia_--while others send haustoria right through the outer epidermal walls--_e.g._ _Erysiphe_--and thus supplement mere contact-irritation and shading by actual absorption from the external cells. Here the fungus is a parasitic epiphyte.