Host and Parasite in certain Diseases of Plants. 



421 



is more permeable and less resistant, and the cell-sap contains a 

 larger proportion of organic acids, glucose, and soluble nitrogenous 

 materials than usual. When the external conditions become more 

 favourable — the temperature higher, the air drier, and the sunlight 

 more powerful — increased transpiration and respiration lead to more 

 normal metabolic activity, for which energetic assimilation provides 

 the materials. Of course, all kinds of combinations are possible in 

 detail, but when dull, cold, wet weather prevails for some time, after 

 & period of bright, hot, and dry weather in the early summer, we are 

 very apt to have herbaceous plants in such a condition as that 

 sketched. 



This being so, I have now to show how the chances of a suitable 

 fungus are increased, if it happens to start its parasitic life on such a 

 host in such a condition. 



Botrytis and other Fungi as Agents of Disease, and their Dependence on 

 the Condition of the Host Plant. 



Let me first proceed to call your attention to a parasitic disease of 

 a, very extraordinary kind, though caused by a fungus belonging to a 

 well-known and widely-spread family. This disease, and the fungus 

 in question, may be met with in nearly every garden and greenhouse 

 all the year round, and is quite common in the open fields and lanes 

 of this country and elsewhere in Europe. In the form generally met 

 with, the fungus has been placed in a separate genus known as 

 Botrytis, though, in the few cases that have been thoroughly worked 

 out, it has been proved that the mould-like Botrytis is only the 

 conidial form of certain higher ascomycetous fungi belonging to the 

 Pezizas, and which agree in developing sclerotia. As we are not con- 

 cerned with the details of the whole life-history of this group, I shall 

 purposely avoid further reference to the higher stages of development, 

 confining our attention chiefly to the Botrytis stage.* 



On dead and dying leaves, twigs, fruits, &c, of plants from all 

 parts of the world, in the open and in greenhouses, in Europe and 

 elsewhere, there is often to be observed an ashen-grey mould, super- 

 ficially not unlike the Phyiophthora of the potato-disease. It appears 

 under various slightly different aspects as regards the shade of colour, 

 the length and degree of branching of the conidiophores, and the size 

 and shape of the conidia, and many different species have been figured 

 and described, some good, many bad, according to the variations in 

 colour, size, &c, referred to, and the substratum on which the mould 

 is found growing. 



It sometimes happens, however, that this same mould is found 



# For further details as to the morphology, &c, cf. de Bary, ' Comp. Morph. and 

 Biology of the Fungi,' &c, especially under the heading Peziza Fuclceliana. 



