W. MCRAE 247 



varies considerably especially in nutrient-agar cultures. 16 is usually from 

 4 to 6fx, but varies from 3 to 10//,. At the place of insertion the wall may be 

 thickened considerably and often quite irregularly inside both the sporangium 

 and the sporangiophore, e.g., PI. II, figs. 6, Sand 9. The sporangiophore may 

 be inflated at various points along its length into large or small swellings, e.^r., 

 PI. II, fig. 11, is a simple example. Usually, however, in vigorously growing 

 cultures, and especially in water-cultures the sporangiophores are regular and 

 uniform as in PI. II, fig. 2, and it is in old cultures that the greatest variations 

 are found. 



The method of formation of zoospores within the sporangium is typical 

 of the genus. The central vacuole is large, its diameter being from 0-2 to 0'4 

 that of the transverse diameter of the sporangium. When the sporangium 

 is ripe it germinates rapidly in the presence of water. Usually the apex of the 

 papilla dissolves and the protoplasm, w^hich has already segmented into 

 zoospores, escape^ through the opening, when the individual zoospores become 

 free from one another and swim away. Usually from 14 to 22 zoospores emerge 

 from a sporangium but sometimes down to 10 and up to 30 and 40 have been 

 counted. In secondary sporangia produced for example on a germ-tube the 

 number is usually small. Though the protoplasm within the sporangium has 

 segregated into zoospores before the opening forms, they as a rule emerge one 

 after another so fast as to appear hke a stream of protoplasm, and they 

 congregate just outside the mouth moving slowly for a few seconds. Then one 

 or two break away and the rest suddenly become active and swim away faster 

 than they can be counted. At other times they come out one at a time 

 very slowly and may even take 10 seconds each to pass through the opening. In 

 the morning when the temperature is lower they are not so active as they are 

 in the higher temperature of the afternoon. As each passes out through the 

 narrow opening it contracts considerably assuming a long dumb-bell shape, but 

 rounds again immediately it passes the opening. When part of the protoplasm 

 has escaped, the part that is still within the sporangium often separates into its 

 individual zoospores which move about within the sporangium, but ultimately 

 escape. Occasionally some of the zoospores do not escape, but remain within 

 the sporangium and germinate there, and the germ-tube usually passes through 

 the apical opening though sometimes through the spoiangial wall. In one 

 case the apex of the papilla swelled up into a thin vesicle into which the pro- 

 toplasm passed from the sporangium, each zoospore becoming separated as it 

 emerged into the vesicle. After a very short interval the vesicle buist and 

 the zoospores swam away. This method of discharge was seen by the writer 

 once only, and has ^not been confirmed though he and his assistants have 



