DISEASES OF PLANTS. 389 



mine the controlling treatment ; but in other cases, where the disorder 

 is strictly of a local character, it may be a simple matter to remedy the 

 trouble. In the case of those peculiar developments of the oak which 

 give us the gall-nuts of commerce, or of similar abnormal develop- 

 ments in the tissues, we have instances of well-defined disease, but it 

 is of a strictly local nature ; the disturbance of functional activity does 

 not extend beyond very narrow limits. It becomes, then, a simple mat- 

 ter to treat the case, because the part may be removed without in- 

 flicting injury upon other organs of the plant, and thus the knife is the 

 sure remedy. Or, again, certain diseases may originate in the break- 

 ing of a limb or the fracture of a surface tissue. In such cases the 

 disease will follow the injury and progress slowly, but it is often a 

 simple and easy matter to prevent its introduction into the general 

 system by properly caring for the wounded part in the first instance. 

 Nature herself provides the means of warding off disease in just this 

 way, and within certain limits her provisions are most effective. If a 

 structure such as a vigorously-growing plant be injured, there at once 

 appears a clear fluid, which gradually thickens into a mucilaginous 

 substance, and finally becomes dry and hard. Under its early protec- 

 tion, a tissue of cork is formed over the wound as a healing and pro- 

 tective structure, impervious to air or water. Under it, the injured 

 parts, now excluded from the air, are able to perfect the healing pro- 

 cess by the formation of new tissue. 



In plants, as in animals, diseases may be developed through a 

 great variety of causes, but it is possible to bring them into a rude 

 system of classification by means of which their consideration is 

 greatly facilitated. The best arrangement of the kind which we have 

 at present, one which answers very well, is that of Hartig,* according 

 to which diseases are developed through the action of — 



1. Phenogamic plants. 



2. Cryptogamic plants. 



3. Injuries. 



4. Soil influences. 



5. Atmospheric influences. 



Under the first head we have to deal with those plants, like the 

 mistletoe and dodder, which grow upon others and draw their nourish- 

 ment directly from them — Whence are truly parasitic. Plants of this 

 kind may contain a certain amount of chlorophyl, but usually possess 

 no true roots ; hence they are not only incapable of drawing nourish- 

 ment directly from the soil, but the^y are also incapable of performing 

 the assimilative functions by which materials for the formation of cel- 

 lular structure are developed, in more than a limited manner if at all. 

 Such plants, therefore, must depend entirely upon the already elabo- 

 rated sap contained in their host«, and, feeding exclusively upon this, 

 the latter must suffer in a degree which is proportional to the devel- 

 * " Lehrbuch der Baumkrankheitcn," p. 6. 



