DISK ALGAE 175 



247. Fertilization is effected by a sperm uniting with 

 the egg in the oogone, usually by passing into the open 

 end of the trichogyne. After fertilization the egg in- 

 creases considerably in size, and forms a cellulose coat of 

 its own. The cells which support the oogone send out 

 lateral branches, which grow up and closely surround it, 

 finally covering it entirely (excepting the trichogyne) 

 with a cellular thick- walled "pericarp. " The whole mass, 

 including the fertilized oogone and its investing pericarp, 

 constitutes the simplest form of spore-fruit (sporocarp). 



248. The further growth of the spore-fruit takes place 

 the next spring by the swelling of the protoplasmic con- 

 tents, and the consequent rupture of the pericarp; the 

 inner portion divides into several cells (the proper fruit- 

 spores), which give rise to zoospores closely resembling 

 those developed from the vegetative cells. From each 

 zoospore a new plant eventually arises. 



There is but one genus (Coleochaete) including a few 

 widely distributed species. 



Laboratory Studies, (a) Scrape off a little of the green, 

 paint-like coating from a flower-pot, a damp wall, or a side- 

 walk plank, and examine under a high power for common 

 Green Slime (Protococcus, etc.). 



(6) Gall-fungi may sometimes be found in Spirogyra and 

 Desmids, and in the leaves of evening primroses, plantains, 

 mints, and some leguminous plants. 



(c) Examine the green plants collected from ponds and 

 ditches for Scenedesmus and Pediastrum. The former may 

 often be found in great numbers on the glass sides of jars or 

 aquaria. 



(d) In midsummer search quiet pools for Water Nets. With 

 a fine scissors cut out a piece of one and mount carefully in 

 water. Study with a low power of the microscope. Some of 

 the coenocytes will be found producing zoospores. Search 

 for young nets forming within the old coenocytes. 



(e) Collect fresh specimens of Sea Lettuce, put into a jar of 



