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 })lants collected from ponds and 

 ditches for Sccnedesmus 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. Stud}^ with a low power of the microscope. Some of 

 the coenocytes will })e found producing zoospores. Search 

 for young nets forming within the old coenocytes. 



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



