RESISTANCE OF PLANTS 385 



So long as the plant is capable of resisting infection, it com- 

 bines the toxins excreted by the fungi with protein substances 

 that it elaborates. This phenomenon may be observed, for 

 instance, in the case of infection with rust. Owing to this 

 capacity of binding toxins, the disease is very often localized in 

 definite tissues, which are frequently separated from the healthy 

 parts of the plants by a cork layer. From this combined state, 

 the toxins may again be liberated by the activity of proteolytic 

 enzymes. 



The penetration of the parasite into the tissues of the plant 

 may be accomplished in various ways, but most often the paths 

 of infection are the stomatal openings on the aerial parts and the 

 root hairs. Infection rarely occurs directly through the surface 

 walls of the epidermis ; for they are usually thickened and besides 

 covered with a layer of cuticle, which is very impervious both 

 chemically and mechanically. When the hyphae of the parasite 

 penetrate into the leaf through the stomatal openings, they 

 come into direct contact with thinner and more permeable walls 

 of the parenchyma. But there exists a group of fungi, the 

 mildews, which have the ability to perforate by means of appres- 

 soria the thick outer walls of the epidermis though covered with 

 the cuticle. Formerly it was assumed that the cell walls of the 

 host plant are dissolved by a specific enzyme, but recent investi- 

 gations (Hawkins and Harvey) have shown that the walls are 

 perforated mechanically by means of the high turgor pressure 

 in the hyphae of the fungi. In the roots, infection takes place 

 through the cells of the root hairs, which possess very thin cell 

 walls. The older parts of the roots, which have shed their root 

 hairs and are covered with a layer of cork, are well protected 

 from the intrusion of the parasite. In young seedlings, the 

 infection frequently penetrates through the crown, i.e., the region 

 between the root and the stem, which Is situated near the surface 

 of the soil. This method of infection may frequently be observed 

 in hotbeds. When the stand of the seedlings is too thick, 

 the walls of the cells are thin and offer little resistance to the 

 entrance of parasites. 



The ability of plants to resist diseases is generally termed 

 "immunity." There are different types and different causes of 

 immunity. Sometimes resistance is based on anatomical adap- 

 tations, for instance, on the presence of a thick cuticle such as 



