SAPROPHYTES AND PARASITES 75 



consists solely of the delicate threads which branch and inter- 

 lace each other, and form a thin white film of mycelium, some- 

 times on one and sometimes on both surfaces of the leaves, to 

 which they adhere by means of little projections, or haustoria, 

 which enter the stomata. From this mycelium arise shorter 

 and thicker threads, consisting of a chain of oval cells, each of 

 which falls away consecutively from the apex, and becomes a 

 conidium capable of germination. It is these fallen conidia 

 which mainly give the mealy appearance to the patches. In 

 former times they were treated as autonomous Mucedines, and 

 were included in the genus Oidium. Later in the year these 

 same white mouldy patches, when examined with a pocket lens, 

 will be observed to be sprinkled with 

 little globose bodies, which are at first 

 yellow and subsequently dark brown 

 approaching to black. These are the 

 perithecia, not larger than small pins' 

 heads, attached at the base by delicate 

 filaments of mycelium (Fig. 43). When fig. 43.- 

 mature, the membranaceous coat splits mr ^,. with ascus and 



' r sporidia. 



irregularly at the apex, and exposes the 



contents, which consist of pear-shaped asci, or sacs, each enclosing 

 two, four, or more elliptical hyaline sporidia, which also are 

 capable of germination and constitute the ascigerous fructifica- 

 tion of the JErysiphe. From this brief description it will be 

 evident that this parasite is entirely superficial, or epiphytal, 

 and that the injury it inflicts is caused by obstructing the 

 healthy action of the leaves and, in a manner, killing them 

 by suffocation. When applying external remedies for plant 

 diseases, such as fungicides, it should first be clearly ascertained 

 whether the parasite is or is not epiphytal, since fungicides are 

 more likely to take prompt effect when application can be 

 made to them direct, and, by removing obstruction, restore the 

 healthy action of the leaves. Our commonest vine disease, 

 which attacks the leaves and fruit, is of the present character, 

 although only the conidial condition is accurately known. 



True endophytal parasites are more varied in their character, 

 and consist primarily of the " rotting moulds " — Fungi which 

 have the habit and appearance of Mucedines, but with a more 



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