Minnesota Plant Diseases. 95 



ive method of healing over wounds and a removal from a dry 

 to a moist atmosphere frequently stimulates the plant to ex- 

 traordinary growth. But it is in opening a new field of attack 

 for invading organisms and other disease-causing factors that 

 such externally caused conditions may prove dangerous to the 

 host-plant. 



Many other plants besides oats are predisposed towards dis- 

 ease during their youth or during the youthful stages of certain 

 organs. The corn plant is attacked by the corn smut only in 

 young growing parts and the fungus cannot invade mature tis- 

 sues. Certain conditions of the youth of plants aid in the at- 

 tack of disease; for instance, the thinness of the tissue skins, 

 and the abundance of food material. It must not be assumed 

 that all plants in their infancy are predisposed towards disease, 

 but there are certain conditions which may in general tend 

 to increase susceptibility towards disease. Perhaps the prob- 

 lems of old age are still more productive of predispositions. 

 Young tissues have the advantage of vigorous protoplasm, 

 while aged plants have reached their limit of growth and are 

 losing ground. Again, seasons may bring predispositions, as 

 in the oat smut, for several causes may contribute to the same 

 predisposition. A rest period may also be productive of dis- 

 ease since the protoplasm is not as active in resistance when 

 resting as in the rapidly growing condition. Such is the case 

 of the ripe rot of fruit. Predispositions of form have already 

 been cited in the thin-skinned potato. Immunity from certain 

 diseases sometimes comes with wax-coated surfaces or thick 

 cork, etc. Physiological habits of plants, as in the oat smut, 

 are likewise productive of predisposition or immunity. Such 

 habits as in the germinating period of grains growing in differ- 

 ent regions may be important in assisting the plant to escape 

 disease. This is the partial explanation of the advantage of 

 selection of seeds growing in certain regions, e. g., northern 

 grown seeds for northern localities. 



Plants kept under peculiar conditions of moisture or tem- 

 perature may acquire a predisposition towards disease. Hot- 

 house plants suddenly planted in very dry conditions are some- 

 times not able to adjust their water evaporating apparatus to 

 the dryer conditions and suffer wilting and accordingly in some 



