ULVACE.E 



5 6l 



the portions still vegetating in the usual mode ; but sometimes the 

 whole endochrome of one portion of the frond may thus escape in 

 the form of zoospores, leaving behind it nothing but a white flaccid 

 membrane. If the microscopist who meets with a frond of an Ulva 

 in this condition examines the line of separation between its green 

 and its coloured portions, he may not improbably meet with cells in 

 the very act of discharging their zoospores, which ' swarm ' around 

 their points of exit very much in the manner that animalcules are 

 often seen to do around particular spots of the field of view, and 

 which might easily be taken for -true Infusoria ; but on carrying his 

 observations further, he would see that similar bodies are moving 

 ivithin cells a little more remote from the dividing line, and that a 



FIG. 425. Formation of zoospores in Viva latissima : a, portion of the ordinary 

 frond ; 6, cells in which the endochrome is beginning to break up into segments ; 

 c, cells from the boundary between the coloured and colourless portions, some of 

 them containing zoospores, others being empty ; d, flagellate zoospores, as in active 

 motion ; e, subsequent development of the zoospores. 



little farther still they are obviously but masses of endochrome in 

 the act of subdivision. 1 



More recent observation has brought out the interesting fact 

 that in Ulva and its allies there are two kinds of swarm-spore, a larger 

 kind, * megazoospores,' with four, and a smaller kind, ' microzoospores,' 

 with two cilia each (see fig. 422). Of these the megazoospores 

 germinate directly, as above described, while the microzoospores or 

 'zoogametes' have been observed to conjugate in pairs, producing 

 zygospores, by the germination of which a new generation is 

 produced. The two kinds of zoospore may be produced on the same 

 or on different individuals. 



1 Such an observation the Author had the good fortune to make in the year 1842, 

 when the emission of zoospores from the Ulvacece, although it had been described 

 by the Swedish algologist Agardh, had not been seen (he believes) by any British 

 naturalist. 



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