260 



PART III. TH CLASSIFICATION OF PLANTS 



The sporangia (and gonidangia) are in all cases unicellular. In the simple 

 filamentous forms they are somewhat enlarged and rounded cells, either inter- 

 calary in position (e.g. Pylaiella), or terminal, occupying the place of a lateral 

 branch, and generally sessile (e.g. Ectocarpus, Choristocarpus, Sphacelaria, 

 etc.). In the more bulky thalloid forms, the sporangia may be merely develop- 

 ments of single superficial cells (e.g. Encceliaceas, Laminariaceas) scattered singly 

 or in groups (sori) over the whole surface. In others again they are borne 

 as lateral branches on hair-like outgrowths from the superficial cells (e.g. 

 Chordariaceas, Sporochnaceae, Stilophoraeeae). In certain cases, where the 

 shoot presents differentiation into stem and leaf (e.g. Chaetopteris, Cladoste 



JJ. 



FIG. 186. Fertile leaves of Cladostephus verticillatus: A sporophyll ; one of the uni- 

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

 bearing the multicellular gametangia. ( x 280 : after Pringsheim.) 



phus), the sporangia are borne on specialised leaves, sporophylls (Fig. 186 ; sec 

 also p. 78). 



The gametangia are in all cases multicellular, each cell constituting a loculu< 

 which gives rise to one or more planogamete*. In their distribution and general 

 morphology they resemble the sporangia. The gametangia 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, Cutleriaceae) ; the small 



