180 SPORE DISSEMINATION 



SPORE EXPULSION AMONG PHYCOMYCETES 



Among some aquatic fungi, such as the Chvtridiales and Sapro- 

 legniales, sporangiospores are merely ejected to the exterior of the 

 sporangium, where, by virtue of their flagella, they become rather 

 widely distributed. Members of these orders generally possess an 

 exit tube or papilla. As the result of increased turgor after de- 

 limitation of sporangiospores, the sporangium opens at the exit 

 tube or papilla, and the sporangiospores are rapidly ejected, either 

 en masse or singly. In Achlya and Aphanomyces they are quies- 

 cent on expulsion and collect in a hollow sphere at the orifice. 

 In Saprolegnia and Leptolegnia they emerge in an actively motile 

 condition. In the related Aplanes they are retained within the 

 sporangium. In Dictyuchus the sporangial content is cleaved into 

 segments, a pore is developed from each segment, and the proto- 

 plast escapes from each segment as a motile spore, leaving behind 

 a reticulum of emprv cells. In Saprolegnia two planetic (motile) 

 stages normally occur, a phenomenon no doubt well adapted for 

 increased dissemination of the species. 



Apparently none of the Peronosporales, except species of 

 Sclerospora, forcibly expels its sporangia. As observed by Wes- 

 ton (1919), S. Philippine mis and 5. gram'inis, occurring on maize, 

 possess a double wall separating the tip of the sterigma and the 

 sporangium. At first these two walls in contact with each other 

 are plane. As the sporangium grows and turgor increases, these 

 membranes tend to bulge outward, and this tendency is restrained 

 by adhesion of the two surfaces in contact. Eventually adhesion 

 is overcome by the stress from increased turgor, and with a sud- 

 den snap both membranes bulge outward, catapulting the spo- 

 rangium away. It can then be caught by air currents and trans- 

 ported to near-by maize plants. 



In Peronospora t abaci na and certain other species of Perono- 

 spora the sporangia are effectively liberated, but by an entirely 

 different mechanism. The sporangiophores grow closely 

 crowded. Each sporangiophore looks like a little tree, and to- 

 gether the sporangiophores constitute a miniature forest with 

 interlocking branches. The entire tree, including its twig tips, 

 sterigmata, is a single, inflated coenocytic cell. A slight change 

 in relative humidity in the immediate environment of the sporan- 



