GROUP 1. THALLOPHYTA : ALG.E. 



267 



loculus which gives rise to one or more planogametes. In their distribu- 

 tion and general morphology they resemble the sporangia. The game- 

 tangia of any one species are, as a rule, all exactly alike, but in some few 

 cases they present two forms which differ in the size, "and consequently in 

 the number, of their constituent cells (e.g. Ectocarpus fenestratus and E. 

 secundus, Cutleriacese) ; the small-celled gametangia are considered to be 

 the male, and the large-celled the female organs. The plants may be 

 monoecious or dioecious (Cutleria). 



The zoospores and the planogametes are generally all very much alike : 

 in Cutleria, however, and in those species of Ectocarpus which have two 

 kinds of gametangia, the one kind of planogamete (female) is considerably 



PIG. 151. Fertile leaves of Cladostephus vertivillatus: A sporopbyll ; one of the uni- 

 cellular sporangia has discharged its zoospores with a mass of mucilage ; B gametophyll, 

 bearing the multicellular gametangia. (x280: after Pringsheim.) 



larger than the other (male), and has a shorter period of motility ; the 

 smaller planogametes are developed in the small-celled gametangia. A 

 sexual process has been observed in but few cases (Ectocarpus silicuiosus, 

 Scytosiphon lamentariiis, Cutleria). In the two former the planogametes 

 are externally similar, but they behave differently in the process of con- 

 jugation, some coming to rest earlier than others, thus indicating that 

 they are female. When the female plauogamete is at rest, it is approached 



