AN AGRICULTURAL REVOLUTION. 



6 43 



the disease its other common name of downy mildew. This fungus 

 reproduces by means of minute particles called spores, corre- 

 sponding in function to the seeds of higher plants. If one of these 

 spores lights upon a moist leaf, its inner contents divide into a 

 number of distinct particles, which soon escape through an open- 

 ing in the spore wall ; then each particle swims about in the film 

 of water for a short time — resembling a 

 little animal — when it becomes quiet and 

 sends out a minute tube which pene- 

 trates the skin of the leaf. It then con- 

 tinues to develop inside the leaf, push- 

 ing about between the cells, and forming 

 the mycelium or vegetative portion of 

 the fungus. As there is little nourish- 

 ment to be found between the cells, this 

 mycelium develops minute processes, 

 which push through the cell walls and 

 absorb the cell contents. A small section 

 of an affected leaf, greatly magnified, 

 is represented at Fig. 5, the unshaded 

 double-walled spaces showing the leaf 

 cells, the shaded part between the walls 

 the mycelium of the fungus, and the 

 projections a, a, the processes or suckers 

 that penetrate the cells. When these 

 vegetative portions of the fungus have 

 developed in the leaf to a certain extent, 



they send out through the breathing pores or stomata their fruit- 

 ing branches, which bear upon their tips the small oval spores 

 (Fig. 6). These fruiting branches form the so-called mildew on 

 the plant, and, as they only appear under certain atmospheric 

 conditions, the mycelium may exist in the vine for some time 

 before this outward manifestation of its presence is seen. On 

 this account a whole vineyard sometimes appears to be invaded 

 by the mildew in a single night. 



From the above description it will be readily seen that this 

 fungus can not be successfully combated after it has established 

 itself within the tissues of the host. To prevent its injuries one 

 must also prevent its ingress to the plant. Fortunately, this can 

 be done by coating the green parts of the vine with some sub- 

 stance having a destructive effect upon the spores of fungi. The 

 salts of copper have such an effect, and in consequence they have 

 come into general use as fungicides. They were first experimented 

 with on a large scale in the vineyards of France, and gave such 

 satisfactory results that they were adopted in a practical way by 

 many growers. In America this use for them has hardly been 



Fig. 6. — Fruiting Branches. 

 Greatly magnified. 



