BREEDING DISEASE-RESISTANT PLANTS 401 
food in similar fashion. Pathogenic fungi and bacteria are wholly or 
partially parasitic. Bacteria which cause plant diseases are those capa- 
ble of establishing themselves and multiplying in number within the 
living tissue of the host. A few of the important plant diseases caused 
by bacteria are “‘fire-blight” of pears and apples, crown gall of many 
fruit trees, grapes and other plants, and the black rot of the cabbage. 
Some fungi, such as rusts and smuts, are strictly internal obligate 
parasites (as distinguished from those obligate parasites which are wholly 
or partially epiphytic), 7. e., they cannot exist outside the body of a 
particular host plant or plants except in the spore stage. In such cases 
the relation between parasite and host is symbiotic. The specific re- 
lations between parasites and their hosts vary from a condition of tol- 
erance of the parasite without scrious injury to the host to one in which 
the destruction of the host finally ensues. Many fungi, such as the 
powdery mildews, are epiphytic although they derive their nourishment 
from the living plant tissue by means of haustoria. Between the epi- 
phytes on the one hand and the internal parasites on the other are many 
types of endophytic fungi in which various proportions of the parasite’s 
life cycle are spent within the host plant. 
Thus there are many agencies, some non-living as well as many living 
things, which threaten the normal development of cultivated plants. 
Even among the parasitic fungi themselves there are many devices for 
invading the host plant and many instances of specific physiological 
relationship between parasite and host. 
The Nature of Disease Resistance in Plants.—Disease resistance in a 
plant may be defined as the ability to develop and function normally 
under conditions such that other plants of the same species fail to develop 
or are destroyed. Resistance is always either partial or complete. The 
avoidance of disease by such means as precocious or delayed maturity is 
hardly to be considered as true disease resistance. Since there are so 
many agencies which may cause disease in plants it is evident that the 
ability to resist disease may depend on any one of many characters or it 
may involve every function of the plant. In either case this ability is a 
manifestation of the physiological individuality of the plant and hence it 
may be inherited. Nowhere is this more strikingly shown than in the 
disease resistance of certain natural species. 
Disease Resistance in Natural Species.—The nature of disease resist- 
ance in a particular instance is indicated by the nature of the cause of the 
disease. In the case of non-living causes resistance on the part of certain 
plants can be explained only as a manifestation of the inherent properties 
of the protoplasm. Thus the alkali resistance of salt grass, the Australian 
salt bushes, the common beet and asparagus is a heritable character. If 
it were not so these species could not perpetuate themselves on soils which 
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