SAPROPHYTES AND PARASITES 685 



g. The chief agent in the distribution of the infecting spores is 

 the wind, but in a few cases insects carry on the work. The 

 entrance of fungi into plants is effected in various ways. In 

 the 'rusts' the germ-tube of the summer-spores chiefly pene- 

 trates through the stomata, but in many of the worst parasites 

 the germ-tubes and hyphae of the mycehum secrete an enzyme 

 which dissolves the cell- walls of the host. 



Several destructive fungi, such as those causing 'canker' of 

 fruit-trees and of larch, find an entrance into their victims 

 through wounds, cracks, and abrasions of the bark produced 

 by frost, gun-shots, unskilful pruning, and other mechanical 

 means. Nutrient substances exude from such wounds and 

 afford an excellent medium for the active growth of fungi, and 

 the walls of the exposed cell are more easily penetrated than 

 those forming the normal outer covering of the plant. Some 

 parasitic fungi confine their attacks to a single host-species, 

 while others are able to destroy many different species of plants. 

 Apart from specific differences of the attacking fungi, peculiar 

 and little understood conditions of the host and its surroundings 

 determine, to a considerable extent whether or no it shall be 

 invaded by a particular parasite. Superabundant moisture, 

 excessive dryness, imperfect access of light and air to the plants, 

 and other external conditions of soil and atmosphere, tend to 

 check their healthy growth and render them susceptible to attack. 



Certain internal conditions of the plants influence the attack 

 of parasites. The experiments of Miyoshi have shown that the 

 hyphae of fungi are able to exert considerable mechanical 

 pressure upon the membranes of the epidermis, when attracted 

 by chemical substances within the cells of the leaf. Moreover, 

 the hyphae of fungi may be induced to grow towards and 

 penetrate into the tissues of plants by injecting the latter with 

 sugars and other substances which exert an attractive stimulus 

 upon the hyphas. 

 This chemiotaxis or stimulating action of certain chemical 



