PHYCOMYCETES 25 



the surfaces of hosts wet with rain or dew. Zoospores are not 

 found in groups above the level of the Peronosporales. They 

 are wholly absent in the Zygomycetes, Ascomycetes, and 

 Basidiomycetes. 



The number of spores formed in the sporangium is almost 

 universally indefinite, but in a few genera is characteristically 

 one or two. In the group as a whole it is extremely various 

 (often several hundred), and is chiefly dependent upon the 

 number of nuclei which chance to flow in from the thallus below. 

 The sporangium in its asexual character, and in the indefinite 

 number of its spores differs strikingly from the ascus. Moreover, 

 while in the ascus the spores are delimited by the astral 

 rays, in the sporangium successive cleavage planes traverse the 

 cytoplasm and in a wholly indefinite fashion finally cut it into 

 bits which are usually uninucleate. These then round up into 

 spores. There remains no intersporal cytoplasm such as charac- 

 terizes the ascus. The essential differences between spore for- 

 mation in sporangium and ascus are indicated by Harper (1899) 

 and Swingb (1903). 



The hypha on which the sporangium is borne is differentiated 

 in many genera as a definite stalk termed the sporangiophore. In 

 some groups, it is speciaUzed in form and may be characteristically 

 branched. 



If the sporangium forms zoospores, it is called a zoospor- 

 angium. If the spores are ciliated, it may be called a swarm- 

 sporangium. Zoospores in some forms escape from the 

 sporangium through a pore or exit tube of definite form and posi- 

 tion. In others, the sporangial wall disintegrates or bursts in 

 an indefinite fashion. After a period of motihty the zoospores 

 commonly assume the spherical form and become enveloped in a 

 definite membrane. Usually they then put out one or more 

 germ tubes and germinate directly into mycelium. In those 

 lower forms in which mycelium is absent, the zoospore merely 

 enlarges and with or without the assumption of a membrane 

 develops directly into the thallus. Aplanospores are chiefly 

 wind borne. They escape from the sporangium usually by 

 the disintegration of its wall and in germination form germ 

 tubes. 



In many cases in the Phycomycetes, especially in the higher 

 groups, there are found asexual reproductive cells which corre- 

 spond in size and form with the sporangia, are unquestionably 



