94 Minnesota Plant Diseases. 



is at present known, ever inlierits a disease — no disease passes 

 directly from parent to tlie offspring plant in the germ cells. 

 Predispositional characters may be inherited but the first dis- 

 ease factor is never inherited. The infection of the host plant 

 may take place so early in life that at first sight there may ap- 

 pear to be an inheritance, but all such cases at present known 

 have been shown to be simply early infections of the host plant. 

 For instance, oat plants are infected in the seedling stage just 

 after the little plant arises from the grain, while in the darnel 

 grass the infection of the host plant takes place inside of the 

 seedling before the seed is ripe — for, as is well known, the little 

 plant is already well developed when the seed is ripe and has 

 been growing for some time previously. That is to say, in the 

 darnel, the plantlet (commonly called the "germ") inside of the 

 grain is already infected with the fungus. But in neither of 

 these cases is there an inheritance of the disease. 



Kinds of predisposition. Predisposition may be therefore 

 of two kinds, the natural and inherited condition of structure 

 and habit due to internal causes, and the accidental or abnormal 

 conditions which are due, not to internal inherited traits, but to 

 the accident of external forces. Thick-skinned potatoes are 

 known to be more resistant towards certain rots than thin- 

 skinned potatoes, i. e., the thin-skinned forms are naturally 

 predisposed in their structure to that disease. Again, oat 

 grains germinate at about the same time when the oat smut 

 spores germinate and hence the young oat plants are predis- 

 posed to smut attacks by their inherited habit of germinating in 

 the spring. Such might be termed a natural predisposition of 

 habit. On the other hand, a wounded plant is predisposed to- 

 wards the attack of wound parasites by an external force as in 

 pruning, or by wounds caused by cattle or deer, or a wagon 

 wheel, and is more liable to such attacks after receiving a 

 wound. 



Again, the transplanting of plants from a dry to a moist 

 climate may predispose such plants to disease. Here the pre- 

 disposition is induced by external factors. It is noticeable that 

 in both of these predispositions of external cause the pre- 

 disposition as in a wound or in transplanting may not in itself 

 bring about serious injury to the plant. Plants have an effect- 



