May 1, 1920 Philippihe Downy Mildew of Maize 109 



tion, the total length of the conidiophore even in abundant dew varying 

 from 260 to 400 ju, although most commonly it is about 340 ju, while in 

 scanty dew, such as occurs in the hot season, lengths of 160 to 200 ju are 

 generally encountered. In either case, however, the greatest width, 

 just below the branches, is from 15 to 26 ju. The sterigmata are con- 

 sistently about 10 /i in length, with a diameter at the base of about 6 fj.. 



The basal cell is invariably present in the mature conidiophore, forming 

 a structural feature which should be emphasized as distinctive (PI. 24, 

 H, J, L). This cell reaches its greatest width at the septum which sepa- 

 rates it from the rest of the main axis and tapers gradually downward 

 throughout its length, terminating in a rounded, slightly swollen foot 

 which is connected by a slender hypha with the internal mycelium 

 through the stomatal pore. The greatest width of the basal cell is usually 

 about 12 ju, but the length varies from the customary extremes in heavy 

 dew (60 to 120 n) to 30 or even 20 /x in a scanty film of moisture (PI. 24, E). 



When fully mature the conidia are most commonly elongate, ellipsoid, 

 elongate ovoid, or rounded cylindric in shape, are thin-walled and 

 hyaline, and have a more or less finely granular content. The tip is 

 broadly rounded and lacks any papilla or other modification, while the 

 base shows an apiculus, a slight thickening and protrusion of the wall at 

 the point of attachment to the sterigma. Wide variations in the shape 

 of the conidia are common, examples being found of all of the types from 

 subspherical, pyriform, or even lemon-shaped to the extremely elongate 

 types which are shown in Plate 25, C-I/. 



A method has been devised by Rosenbaum {18) for expressing quanti- 

 tatively the shapes encountered in a study of large numbers of conidia 

 of Phytophthora. This method, which consists in classifying and plot- 

 ting the ratios of length to width, is of value in that it gives a quantitative 

 idea of the relative predominance of certain shapes of conidia in a species 

 and furnishes a reliable basis for comparison with others. Unfortunately, 

 however, this method can make no distinction between conidia which are 

 ovate or obovate, pyriform, or obpyriform, elHpsoid, allantoid, or cylin- 

 drical, provided their length and greatest diameter be the same. There- 

 fore, while the ratios of length to width of 400 conidia of the PhiHppine 

 Sclerospora of maize are presented here in tabular and diagrammatic 

 form for comparison with other species, a clearer idea of the variations in 

 shape is probably to be obtained from the figures in Plate 25. 



The size of the conidia also varies greatly. When large numbers are 

 examined, examples are found with such widely different dimensions as 

 to include those given for several other species. It is difficult, therefore, 

 to give a correct impression of the size of the conidia by means of the 

 extreme dimensions within which they vary, or even by means of the 

 average dimensions. However, the method of grouping together large 

 numbers of representative conidia into a series of measurement classes 

 and plotting curves to show their frequency of occurrence has been used 



