2o6 AGRICULTURE OF MAINE. 



higher plants, has definite functions to perform that may differ 

 from those of other tissues of the same plant. The tissues of 

 higher plants are protected externally by a layer of cells called 

 the epidermis surrounding the entire plant. Many little open- 

 ings called stomata permit the interchange of gases to and 

 from the plant. 



The application of the spores or other parts of the parasitic 

 organism to the host plant is called inoculation. When the 

 parasite has established itself upon or within the tissues of 

 the host we say infection has taken place. The manner of 

 infection is interesting and it differs witli different organisms. 

 In the case of the fungus causing bean anthracnose the summer 

 spores germinate upon the surface of the bean plant and 

 produce, at first, a roundish body with one surface closely 

 appressed to the epidermis of the host. From its appressed 

 side an infection tube is sent out which penetrates directly the 

 epidermal cell, and, branching profusely, threads are sent into 

 other cells whose contents are killed and absorbed by tlie 

 fungus. Enough cells are finally destroyed so that a spot is notice- 

 able at this point and we say the plant is diseased. The pene- 

 tration of the wall of the cells by the germ-tube and the fungus 

 hyphae is made possible by the secretion of an enzyme from 

 their tips which dissolves the cellulose cell wall at that point. 

 Rust spores germinate in much the same way except that the 

 germ tube of the uredo spores enters the host plant through 

 the stomata. The mycelium of the rust does not penetrate 

 the cells, but instead little branches or suckers, called haustoria, 

 are pushed in and these absorb from their contents the food 

 necessary for the nutrition of the fungus. In this case the 

 host cells are not at first killed, but continue in a healthy condi- 

 tion as the fungus does not secrete poisonous enzymes nor does 

 it absorb so much nutrition from the host cells as to inhibit 

 their function during the earlier period of the growth of the 

 plant. 



In many cases the parasite is not at all times able to gain 

 entrance to the host plant. Favorable conditions must obtain 

 for the germination of the spore and the entrance of the germ- 

 tube if infection is to take place. Such conditions are not 

 the same for all fungi. The temperature factor may vary, but 

 for the most part, moisture in the form of rain, dew, or 



