ON GEAPE MILDEW. 135 



tor produces decay in dead wood. Many experiments also seem 

 to prove that the cause of the disease is to be found in the 

 fungus, according- to which the fiirtlier diffusion of the evil is 

 greatly repressed by the removal of the first affected shoots on a 

 wall, the destruction of the fungus through washing, &c.* 



The indisputably contagious character of the disease is also 

 explained only by the fact that the fungus causes the malady, for 

 it is easily imagined how the lightest breeze may carry the innu- 

 merable quantity of spores which it produces, and which are 

 only Tu-o"'" "^ length, from the diseased to the sound vines. 



The appearances which the diseased vines present are as fol- 

 lows. The spots on the green bark of this year's shoots on 

 which the fungus has begun to vegetate are discernible by a 

 faint cloud in their green tint before the fungus itself is visible 

 to the naked eye. The fungus consists at this time of minute 

 extremely delicate threads, visible only through a good lens, 

 resembling those of a spider, which creep on the surface of the 

 cuticle, forming an irregular web. The bark has assumed in 

 the spots which are attacked, frequently not exceeding 1'" in 

 diameter, a somewhat deeper tint ; these spots soon increase in 

 diameter as the disease spreads, become confluent, and change, 

 in consequence of the death of the superficial cells, into a choco- 

 late brown. Microscopic observations show that the depravation 

 of the juices which produces this change of colour, and tlie death 

 of the diseased cells, is confined to tlie most superficial strata, 

 while those which lie deeper, as well as the wood, remain per- 

 fectly sound. Under these circumstances the malady, as far as 

 it attacks the branches, seems a very unimportant evil, and there 

 is no danger of the death of the vines which are attacked, inas- 

 much as the external coats of the bark must, in the natui'al 

 course of things, be dried up in the ensuing autumn and winter, 

 and tlirown off during the next year. The fungus exercises a 

 still smaller influence on the leaves than on the branches ; at 

 least I could not remark, even in those vines which were thickly 

 covered with the fungus to the very tips of the shoots, that the 

 vegetation of the leaves was really affected. 



As regards the fruit the case is far otherwise. Here also only 

 the extreme layer of cells at first suffers under the attack, while 

 the inner parts of the fruit, as far, at least, as may be concluded 

 from microscopic observation, remain perfect. The appearances 

 which the affected berries exhibit differ much, according to the 



* I could uot institute the experimentum crncis of banging diseased bunches 

 ■which I bad brought to Tubingen, where the malady has uot yet intruded, 

 on untainted vines, with the view of proving whether it could not be com- 

 municated by contagion, because I would not subject myself to the respon- 

 sibility of contributing to spread the evil in a wine district. 



L 2 



