THE FUNGI : ARCHBIYCETES AND PHYCOMYCETES 233 



Control of Downy Mildew and other Mildews is generally effected by 

 covering the plants with some substance which will kill the Fungus without 

 damaging the host tissues. In recent years great advances have been made 

 in the study of sprays and spraying machinery. Two chief types of sprays 

 are emploved, the one in which a substance or substances are dissolved in 

 liquid and sprayed on to the foliage with the idea of covering the whole 

 surface of the host with a film of the solution. The second is by the use of 

 extremely fine powders which are blown over the plant, relying on the damp- 

 ness of the leaves to retain them. In the former case a great proportion of 

 the liquid is lost, and what is known as a " spreader " is now employed to 

 increase adhesion of the spray to the leaves. This may take the form of an 

 oil, the whole solution being worked up into an emulsion. Where Downy 

 Mildews are concerned the solution known as Bordeaux mixture is generally 

 employed. It consists of a dilute solution of copper sulphate and lime, 

 sometimes with the addition of paraffin oil. 



/ The Evolution of the Conidiospore 



While it is obviously unwise to draw too rigid conclusions about the 

 evolution of a structure from a study of a limited number of existing types, 

 the genus Pythium provides a number of examples in which the asexual 

 reproductive organs apparently show a transition from a simple zoosporangium 

 to a true conidiospore (Fig. 226). It must be realized, however, that in 

 citing the following species as stages in such a series the reproductive organs 

 may in certain instances germinate by different methods under differing 

 conditions, and that many species of Pythium exhibit both zoospores and 

 conidiospores, the former being produced under wet and the latter under 

 dry conditions. 



In the genus Phytophthora, which is included in the family Pythiaceae, 

 the zoosporangia develop from the vegetative mycelium and form zoospores 

 which are kidney shaped and escape directly from the sporangium without 

 the formation of a vesicle. Such a condition is not found in any species of 

 Pythium, for this is one of the characters used to separate the genera. In 

 Pythium gracile, however, the zoosporangia are filamentous and differ but 

 little from the ordinary hyphae. Each, however, produces a long, narrow 

 vesicle which terminates in a spherical swelling and resembles a zoospor- 

 angium in appearance. It is this swollen sporangium-like structure which 

 grows out of the host cell and in which the zoospores are developed, and from 

 which they are finally liberated as reniform, biflagellated bodies. 



In Pythium deharyamim, as we have seen, the zoosporangia are spherical 

 in shape and develop a vesicle into which the contents of the zoosporangium 

 pass and in which the zoospores are differentiated. Under dry conditions, 

 however, conidiospores are formed which are similar in shape and size to the 

 zoosporangia. In fact, we may consider that these conidiospores are really 

 zoosporangia which form no zoospores but germinate by a germ tube. 



In Pythium intermedium both zoosporangia and conidiospores occur. 



