May 28; 1915] 



SCIENCE 



779 



channel are protected against this means 

 of infection. 



The habitus of a plant may influence 

 its disease resistance. An instance of this 

 is the potato with reference to the late 

 blight {PhytophtJiora infestans). Infec- 

 tion of the potato vine with this disease is 

 caused by the conidia being carried to the 

 leaves by the wind. The conidia remain 

 on the leaves until a drop of water causes 

 them to liberate their zoospores. These 

 swim around in the water for some time, 

 then drop their cilia, germinate, and send 

 a hypha into a stoma. Passing through a 

 potato field shortly after a heavy rain, it 

 will be observed that the leaves of some 

 sorts dry within half an hour, while others 

 remain wet for several hours. Generally 

 the quick-drying varieties are less suscept- 

 ible to the disease than the slow-drying 

 varieties. Slow drying is the result of the 

 I)lant's habit of growth, which hinders the 

 evaporation of the rain drops. Such 

 plants have flat leaves. Small, hairy 

 leaves, as well as an airy, open growth of 

 the whole plant, facilitate drying. It is pos- 

 sible that the arrangement of the stomata 

 also may exert an influence on the attack of 

 the fungus. 



In the case of the grape leaf the ar- 

 rangement of the stomata is of great impor- 

 tance. For a long time it was not known 

 why spraying with Bordeaux mixture did 

 not, in all cases, prevent the attack of Pero- 

 nospora. Finally, however, Ruhland and 

 Miiller-Thurgau explained this by show- 

 ing that in the grape leaf the stomata are 

 formed only on the under surface. Spray- 

 ing of the grape, therefore, can be effec- 

 tive only when the spray mixture reaches 

 the under surface of the leaves, and this 

 fact must be borne in mind when dealing 

 with fungi which enter the leaf through 

 the stomata. A similar thing was ob- 

 served by the writer's assistant, Dr. 



Pietsch, whose investigations have not yet 

 been piiblished. He found that the resist- 

 ance of some Remontant carnations is due 

 to the form of the stomata, which makes it 

 impossible for the hyphae to penetrate 

 them. In some cases, however, the hyphffi 

 can not produce infection even though 

 they penetrate the stomata. In the case 

 of cereals immunity from rust is independ- 

 ent of the stomata. 



In cruciferous plants the water pores 

 are the avenues of entrance for many bac- 

 terial diseases. The relation between their 

 form and disease resistance, however, has 

 not yet been established. 



As may be seen in the ease of the potato, 

 the lenticels as well as the stomata may 

 influence immunity. The scab fungus 

 (Oospora scabies) after penetrating into 

 the outer layers of the potato establishes 

 itself in the lenticels and causes the sur- 

 rounding tissues to produce an abnormal 

 corky growth. Bacteria also may enter 

 the lenticels, especially when on account 

 of moist conditions the tissues are form- 

 ing callus. This callus, however, does not 

 form a sufficient protection, and softened 

 tissue and even decaying spots result. 



The lenticels are developed very differ- 

 ently in different varieties of potatoes, and 

 it is therefore important that the relations 

 between them and resistance to scab and 

 bacterial rot be investigated. 



The condition of the cuticle may influ- 

 ence infection, as shown by the behavior 

 of cereal seedlings in resisting smut dis- 

 eases. Such influence, however, is possible 

 only in the very early stages of the seed- 

 lings' growth, that is before the tissues 

 have attained full development. Since 

 the germination tubes of smut are able to 

 dissolve cellulose, there must be stored sub- 

 stances which cause resistance, and in this 

 connection silicic acid is probably the first 

 to suggest itself. Indeed the quantity of 



