juiyis, I9IS Rheosporangium Aphanidermatus 283 



what roughly undulated wall, averaging from 1.5 to 2.^11 in thickness. 

 They have an average diameter of from 17 to ig^i (PI. XLV, fig. 7). 



Germination of the oospores was observed several times in the course 

 of the studies. Water cultures in Petri dishes containing young steril- 

 ized sugar-beet seedlings were employed for growing the spores, which were 

 produced literally by the thousand. After their development the 

 cultures were allowed to dry out slowly without removing the covers from 

 the dishes. When they had remained apparently air-dry for one month, 

 sterile water was added to certain of them and they were observed for 

 evidences of growth. In three out of five cultures zoospores were seen 

 on the second day, but microscopic examination. failed to demonstrate 

 the presence of germinating oospores. While it hardly seems possible 

 that the vegetative . mycelium or asexual spores of the fungus would 

 survive this drying on a glass plate for a month, there is reasonable 

 ground for doubt as to the source of the growth. On June 15 similar 

 cultures started in November and in January were tested for oospore 

 germination in the same way. During the following three or four days, 

 four out of five cultures yielded positive results which could be confirmed 

 by microscopic examination. Of the many thousand spores present, 

 however, only a very few germinated. These invariably put out germ 

 tubes, which developed into vegetative mycelium bearing the character- 

 istic asexual fruiting bodies. 



When the available food supply has been consumed or the water has 

 become sufficiently exhausted by gradual desiccation, aquatic cultures 

 supplement by a further effort at presentation the fructifications already 

 discussed. The cytoplasm which has not been used in spore formation 

 collects in masses in different parts of the hyphae, and walls itself off. 

 These accumulations are most frequently found at the ends of threads, 

 where they appear like small presporangia (PI. XLV, fig. 6) ; but they 

 occur also in other positions. They are more resistant to desiccation 

 than ordinary hyphge and doubtless ser\^e under natural conditions to 

 carry the organism through brief periods of drought. 



DETAILED MORPHOLOGY OF THE ORGANLSM 



For the purpose of clearing up certain important details of the pro- 

 cesses discussed in the preceding paragraphs, material from water cultures 

 at various stages of the life history was killed in Flemming's weaker 

 solution diluted with water, and embedded in paraffin, then sectioned and 

 stained with Flemming's safranine, gentian violet, and orange G. For 

 cilia the gentian alone was used. Camera-lucida drawings from this ma- 

 terial are used to illustrate the following discussion. 



The slides show that the nuclear divisions which precede oospore pro- 

 duction occur only in the older portions of the hyphae (PI. XLIV, fig. 2, 

 3> 4> 5) 8, 9, 11). Following karyokinesis the daughter nuclei are carried 



