308 



Ulotrichales 



growth may, however, be resumed by the penultimate cell assuming the 

 function of an apical cell and growing upwards through the base of the old 

 zoogonidangium, which it leaves behind as a sort of collar. The zoogonidangia 

 are mostly ellipsoid or ovoid, usually sessile, but sometimes stalked, and they 

 open by a terminal or subterminal pore. 



Brand ('10) states that there are three types of zoogonidangia differentiated as follows : 

 (1) the sessile zoogonidangium, which may be terminal, lateral or intercalary, and is 

 without any marked annular thickenings on its dividing-wall ; it arises from a vegetative 

 cell, is never detached from the filament which bears it, and discharges its zoogonidia 

 in situ ; (2) the stalked zoogonidangium, which is cut off from the end of an outgrowth 

 from a vegetative cell and is therefore either terminal or lateral. It has concentric rings 

 of thickening on its dividing-wall, and these ultimately become a mechanism for its 

 complete detachment, which takes place before the zoogonidia escape ; (3) the funnel 



Fig. 201. Three apical cells of Trentepohlia Montis-Tabulss (Reinsch) De Toni var. ceylanica 

 W. & G. S. West showing a series of apical caps (ap. cp.). In C the serrated fringe of a 

 loose cap is well shown, x 800 (after West & Hood). 



zoogonidangium, which is always terminal on a branch. The upper part of the terminal 

 cell becomes funnel-shaped by the formation of a subapical constriction and the dilation 

 of the apex, after which it is cut off by a wall on which two annular thickenings are 

 developed, one over the other. These ring-like thickenings eventually cause the de- 

 tachment of the zoogonidangium before the escape of the zoogonidia. 



In Phycopeltis the zoogonidangia are borne singly on short stalks of one 

 to six cells, which stand erect from the disc, although the actual supporting- 

 cell may be hooked. In Cephaleuros they occur mostly in small clusters at 

 the ends of erect cellular hairs. On becoming wet they rupture and set 

 free the biciliated zoogonidia, but as in Trentepohlia they are frequently 

 themselves detached and even distributed by wind before the swarm-spores 

 escape. 



Many zoogonidia arise in a zoogonidangium. They are ovoid or pear- 

 shaped, with two cilia, or in some species of Trentepohlia with four. It is 



