252 E. J. BUTLER. 



of the parasite arise. These are of two types, sporangia and resting 

 conidia. 



The sporangia are formed terminally on the main branches 

 of the mycehum or on short or long stalks borne laterally on these. 

 They are produced in great numbers in favourable conditions, such 

 as when a piece of leaf-sheath with a spot containing vigorous in- 

 ternal mycelium is incubated in a moist atmosphere for 24 hours. 

 Then a dense growth of hyphse occurs from the stomata, the hyphse 

 being irregularly branched and usually terminating in sporangia, 

 while others may arise on lateral branches lower down (plate IV, 

 fig. 3). The superficial growth in these cases is woolly, from free 

 branching of the emergent hyphse, and is partly erect, partly creep- 

 ing. Sporiferous hyphae may stand up nearly J inch from the sur- 

 face of the leaf. In watch-glass cultures from young infected leaves, 

 long threads consisting of a number of hyphae have sometimes ex- 

 tended from the leaf to the sides of the glass, and on these lateral 

 sporangia are common. Old mycelial webs immersed in water for 

 24 hours often give rise to numerous sporangia on short stalks aris- 

 ing from any part of the mycelium (plate IV, fig. 4). As is the case 

 generally in Pythium, definite conidiophores distinct from the rest 

 of the mycelium, such as are found in most of the Peronosporaceae, 

 do not occur ; the sporangia are formed on the tips of ordinary 

 hyphae or on lateral branches from them. 



The sporangia are very variable in size and shape. In the pre- 

 vious description they are given as 50 by 35ya on an average (ex- 

 tremes 38 to 70 by 33 to 42), but smaller ones than this are not un- 

 common in poor cultures. The shape is typically piriform, the 

 attachment being by the broad end. Irregular shapes, such as those 

 figured in plate IV, fig. 5, are common, and the attachment is some- 

 times lateral (plate V, fig. 10a). When ripe, the narrow end is always 

 papillate, the papilla varying from a small knob to a protrusion 

 1 the diameter of the sporangium ; it is broader than is customary 

 in the genus. 



Germination of the sporangia takes place very rapidly when sown 

 in water, within half an hour under favourable conditions. Four 



