Relation oj Funs^i lo oilier Ori^auisins. .1. Lorrain Siuilh. 21 



nur typical ot parasilisni gencralK'. It has, un l\\c cijiilrarv, 

 been frequently observed that leaves attacked by fiin^i 

 become more bright! v green shewing thus increased 

 assimilation, and transj^iration is generally stronger in infect- 

 ed than in healthy leaves. There is also frecjuently a certain 

 prolonged resistance, a reaction of the host plant that indicates 

 increase oi vitality due to toxic excitation by the fungus. 

 There may be an abnormal growth and increase by division of 

 the neighbouring cells so that a barrier of new tissue is 

 often interposed against the invader. Thus in the attack 

 of Primus leaves by C^ylindrosporiiiin, Higgins* has described 

 how the lu'ph.-e of the fimgus, gaining entrance by the 

 stomata, travel towards the epidermis, generally of the upper 

 surface where a thin stroma is built up which gives rise to 

 the spores. Meanwhile beneath the stroma there is formed 

 b\- the host a layer of much swollen cells which constitute 

 a separation layer, so that in time the part of the leaf affected 

 breaks away at the region of these cells leaving a " shot- 

 hole." The swelling of the cells was found by Iliggins to 

 to be due to the breaking down of the amygdalin molecule 

 of the host cells into smaller molecules by the emulsin of 

 the fungus thus greatly increasing the pressure. As a result 

 of the dropping out of the affected area, the fungus secures 

 dispersal, but the leaf also benefits by casting out the 

 diseased portion. 



Another aspect of host reaction takes the form of 

 " immunity " of varying degree to fungus attacks. 

 Stakmanf and others consider that the success of infection, 

 including the successful development of the parasite, depends 

 on the degree of symbi(xsis between the two organisms. In 

 plants susceptible to fungus attacks, the hyph,:^ grow 

 vigorously without immediately affecting the host cells to 

 any great extent. In resistant forms the fungus gains 

 entrance, but the cells in the neighbourhood are at once 

 disorganized and killed and further penetration bv the 

 fungus is entirely hindered. The degree of resistance is 

 thus commensurate with the rapidity of action. The hyphre 

 eventually die off either from lack of suitable nourishment, 

 or possibly, Stakman thinks, there may be some very definite 

 antagonism between the cells of the immune plant and the 

 fungus which requires further explanation. Immune plants 

 are often varieties or strains of susceptible species, and this 

 peculiarity, as in the case of wheat, potato and other economic 

 plants, is of very great importance in combating disease. 



* Higgins, '14. t Stakman, '15. 



