SLIME MOULDS (mYXOMYCETEs) 1 3 



kept a Plasmodium in a streaming condition for over a month be- 

 neath a bell jar. Physarum psitiacinum, which inhabits the rotten 

 stumps of old trees, appears to pass a year as a plasmodium. 



The early stages in the formation of the sporangium have been de- 

 scribed in Comatricha obtusata. When the fruiting period is reached, 

 the watery- white plasmodium issues from the wood crannies and spreads 

 over an area perhaps half an inch across. The plasmodium is seen to 

 concentrate in thirty or forty centers and in an hour or two each 

 center has by rhythmic pulsation of the protoplasm risen into a pear- 

 shaped body with a slender base and an enlarged upper portion. The 

 black hair-like stalk has grown to its full length in six hours and on 

 its summit is borne the young sporangium, which is a white viscid 

 globule of protoplasm. A pink flush now begins to appear in the 

 sporangium. The included nuclei are like those of the plasmodium 

 at first, but later as spore formation proceeds they divide mitotically. 

 The sporangia of the different slime moulds take various forms which 

 will be described in general in the systematic generic keys which 

 follow. They may be either symmetric or irregular in shape, sessile or 

 stalked. The irregular sessile forms, which simulate the net-like 

 appearance of the streaming protoplasm, are called plasmodiocarps. 

 When the fruit body is flat and cake-like with separating walls imper- 

 fectly developed it forms an csthalium. The protoplasm which is 

 left on the substratum and dries down as a film-like residuum is known 

 as the hypothallus (Figs. 2 and 3). 



The changes which take place in the formation of spores and 

 capillitium have been minutely studied in a number of slime moulds. 

 We owe much to R. A. Harper, E. W. Ohve and B . O. Dodge in America 

 and to E. Jahn in Germany for our knowledge of these processes. The 

 process in Didymium melanospermum, according to Harper,' is as 

 follows: The spore plasm condenses so that it is finely granular in the 

 peripheral region and central region near the columella and foamy 

 vacuolar in the middle zone. The capillitium is already formed before 

 the condensation of the protoplasm has been accomplished. It con- 

 sists of smooth threads which pass radially outward from the central 

 dome-shaped columellar cavity to the sporangial wall. The threads 

 of the capillitium are attached at their ends. The protoplasm is in 

 contact with these threads and at this stage the nuclei are scattered 



' Harpkr, R. a.: Amer. Journ. Bot., i: 127-144, March, 1914. 



