MILDEWS. 47 



swelling. A passage is now formed by which the living matter 

 of the smaller branch passes over and fuses with that of the larger. 

 After this act of impregnation the blended mass contained in the 

 larger branch develops a thick dense wall about itself, and assumes 

 the character of a mature winter spore. In this condition it can 

 successfully survive extreme and long continued cold or drought, 

 and can, on the recurrence of favorable conditions, germinate and 

 infect fresh grape leaves. The fungus so produced soon develops 

 summer spores, which may infest surrounding vines at a rapid rate, 

 and in its turn form winter spores towards the end of the season. 



This is the story of the grape vine mildew. Leaves which are 

 attacked rapidly wither, lose their power of providing food mate- 

 rial for the plant, and at length fall off. Thus deprived of its 

 proper food supply, the plant is seriously' weakened, and while it 

 may produce new leaves these may in their turn be attacked. But 

 even if they do not suffer, the suspension of normal activity for a 

 considerable period during the growing season is a check from 

 which the vine cannot wholly recover, and those which have been 

 seriously infested are always less mature when growth ceases, and 

 so less able to withstand the severity of the following winter. 

 Besides this, the fruit stems, and even the berries themselves, may 

 be attacked, and the crop thus greatly reduced or even practically 

 destroyed. This fungus is a native of America and was introduced 

 with American vines into European vineyards about 1877. There 

 it has spread very rapidly and has proved far more fatal in its 

 effects on many of the delicate European varieties than on our 

 own species, which may, perhaps, be supposed to have become 

 somewhat inured to its attacks by generations of exposure and 

 selective survival. 



Another of the mildews equally destructive with that of the 

 vine, and so equally important in an economic point of view, is 

 that which causes the so-called rot and blight of the potato plant. 

 The threads of this fungus penetrate the stems, leaves, and tubers 

 of the host, causing the rotting of the tubers and, in severe cases, 

 the complete collapse and death of stems and leaves. A casual 

 glance does not detect the disease until it is far developed, but 

 careful examination during its early stages will show the usual 

 discolorations of the leaves at the points where, on their under 

 surfaces, are found the downy tufts of branching threads which 



