1890.] Host and Parasite in certain Diseases of Plants. 215 



protoplasm can dispose of the small amounts of poison coming in 

 from the hyphas, by respiratory oxidation or otherwise, the hypha is 

 debarred access to the cells, but immediately the poison succeeds in 

 lessening or destroying the power of the protoplasm to control the 

 cell-sap, the latter exudes through the permeable protoplasm, and 

 suffuses the whole tissues with acid sap, containing just such food- 

 materials as the fungus flourishes in. Consequently the latter 

 spreads quickly, killing the cells more rapidly than ever, and soon 

 destroying large tracts of tissue. This killed tissue turns brown, 

 and we can consequently trace the progress of the disease by the 

 spread of the discoloration. It was shown that the destructive power 

 of the fungus concerned — i.e., the capability of its hyphae to produce 

 the poisons — can be enhanced by culture in solutions of sugar, organic 

 acids, and a little nitrogenous material and salts — just such a solution 

 as is obtained in infusions of dead vegetable tissues ; consequently 

 the destructive power of the parasite increases as it feeds on the pro- 

 ceeds of destruction. 



It has recently been discovered that the successive crops of spores 

 of the fungus differ in infective power, and that, whereas the spores 

 first formed may be unable to infect a living plant, those of the second 

 or third generation can do so. 



In conclusion, and passing over the observations and references 

 to other diseases, there are four chief points to be considered in 

 regard to the epidemic fungus-diseases concerned. 



First, there is the healthy host-plant itself, which may be a more 

 or less favourable object for the fungus. Secondly, there is the 

 fungus, which may, or may not be, able to kill the living cells of the 

 host. Thirdly, the influence of variations in the environment — 

 especially low temperatures, want of light, and damp air — may so 

 affect the host-plant that it is more easily and quickly infected by the 

 fungus than was the case when its cell- walls were thicker and harder, 

 and its protoplasm more capable of effecting certain processes of 

 metabolism, and controlling the sap in the cells. Fourthly, the 

 fungus also is capable of being rendered more formidable by varia- 

 tions in its environment, and especially by invigorating culture in 

 suitable food materials. 



Now when the external conditions are such that they favour the 

 development of the fungus, while at the same time lowering the 

 metabolic activity and respiratory power of the protoplasm, the 

 conditions for an epidemic of the disease in question exist, and 

 this is frequently realised in a dull, cold, wet July or August in 

 this country. The point especially insisted on is not that any- 

 mysterious predisposition to disease is here manifest, but that the 

 one plant — the fungus — is favoured by the prevailing conditions of 

 culture more than the other— the host-plant. If we wanted to culti- 



