A Study of Meadow-Crop Diseases in New York 49 



germination of the spores in the clover extract on two slides was 19 per 

 cent. Apparently the clover material stimulates the germination. The 

 effect may be due to carbon dioxide acting on the pH of the water, as shown 

 by Platz, Durrell, and Howe (1927) for Ustilago zeae, but this was not 

 investigated further since the stimulation was not great. 



The conidia may germinate within twelve hours, usually pushing out a 

 germ tube terminally, sometimes laterally. The germ tubes grow slowly 

 in distilled water and show signs of disintegration after about forty-eight 

 hours. The protoplasm becomes vacuolate and hyaline. In clover 

 extract the tubes become somewhat longer and die somewhat less quickly 

 than in distilled water, but none the less surely. The protoplasm remains 

 in the end of the germ tube. This was observed in one case as the 

 protoplasm flowed from the spore to take up this position in the tube. 



The first hint of mycelium on the leaves in the greenhouse may be 

 detected within four days after inoculation. The patches of mycelium 

 increase in diameter and fuse, producing within two weeks small necrotic 

 spots which enlarge, finally coalesce, and kill the leaf. 



The most marked pathogenic effect of the mildew fungus on the clover 

 plant is stunting. Some data bearing on a possible explanation of this 

 effect are given in table 17. There was a marked difference between the 

 size of the mildewed plants and the unmildewed ones when they were 

 harvested. A glance at table 17 reveals that the disease-free plants averaged 

 1.73 times heavier than the diseased ones when green and 1.53 times 

 heavier when dry. It appears therefore that the mildew exercises a pro- 

 found deleterious effect upon the economy of the clover plants. The 

 decrease of the ratio between the two with drying indicates clearly that 

 the diseased plants contain less water than the healthy ones, both upon 

 an absolute and a per-gram-dry-matter basis. This is borne out by 

 the data in table 17 which show that the treated plants average 5.39 grams 

 of water per gram of dry matter and the untreated averaged 4.65 grams. 

 The conclusion obviously is that the untreated plants were living under 

 physiologically dryer conditions than were the treated plants. 



Professor H. H. Whetzel in conversation with the writer has suggested 

 that the stunting of mildewed plants is due to the dry condition that has 

 been shown to exist within the tissue and that this dry condition is due 

 to excessive transpiration of the diseased plants. The water is lost rapidly 

 from them through the thick mat of hyphae spread over the leaf. These 

 cylindrical hyphae expose much more surface to the dry air than does the 

 flat leaf surface they cover, even if that were uncuticularized and free of 

 hyphae. They lose water so rapidly that they can hardly be removed from 

 the leaf quickly enough for microscopical examination without shrivelling. 



It is well known, of course, that high transpiration causes the following 

 effects: the stomates tend to remain closed, keeping out C0 2 ; the dry walls 



