MILDEWS. 45 



follow its individual development. A good type for this purpose 

 is one of the most troublesome members of the group — the mil- 

 dew of the grape vine. The white dusty patches produced on the 

 iower sides of the leaves and the corresponding discolorations of 

 their upper sides, are only too familiar to every owner of grape 

 vines. They appear most commonly in midsummer, and the dis- 

 -colorations usuall}' give the first warning of the presence of the 

 disease. If, at this time, a thin section be cut through the thick- 

 ness of the leaf, the vegetative organs of the fungus will be found 

 pushing their way among the more or less closely packed cells of 

 which the leaf is composed. 



This vegetative body or thallus of the fungus consists of fine, 

 colorless, branched threads occupying the spaces between the cells 

 and at intervals sending globular absorbing organs into the cell- 

 interiors. By means of these absorbing organs the food-materials 

 prepared in the cells of the leaf are appropriated by the fungus, 

 to the direct injury of the grape vine. The growth of the fungus, 

 thus bountifully supplied with food ready prepared, is naturally 

 rapid and luxuriant. 



The cells near the under surface of the leaf are quite looselj'^ 

 arranged, leaving considerable air spaces which communicate with 

 the outer air through special openings, or breathing-pores. As 

 the fungus develops, the ends of numerous threads emerge either 

 singly or in tufts, through these pores, standing vertically to the 

 under surface of the leaf. Toward their ends, these threads 

 branch freely into a tree-like form, the tip of each branch being 

 drawn to a point. At each tip now appears a swelling which en- 

 larges till the whole structure has the aspect of a tiny tree, with a 

 fruit at the end of each twig. When fully formed these bodies, 

 which are pear-shaped, are readily detached and great numbers of 

 them are carried away in tiny clouds of dust by light winds. Each 

 of them is now an independent cell, consisting of a mass of living 

 matter surrounded by a delicate membrane, and about the one- 

 thousandth of an inch in length. It is natural to suppose that they 

 serve to reproduce the fungus, and investigation justifies the sup- 

 position. They may be known by a term commonly applied to 

 nearly all the reproductive bodies of those plants which do not 

 produce flowers, namely, spores. The term is a very general one 

 and includes structures of the greatest diversity, which, however, 

 perform for their respective parent plants the same office which is 



