72 



FIKST GROUP.— THALLOPHYTES. 



spring from the cells that line the conceptacle. Many species are monoecious ; both 

 kinds of sexual organs are formed in the same conceptacle, as in Fuc us platy carpus 

 (Fig. 44) ; others are dioecious, the conceptacles of one plant containing only oogonia, 

 of another only antheridia, as in F. vesiculosus, serratus, nodosus, Hitnanthalia lorea. 

 Between the sexual organs in the conceptacles are numerous unbranched, long, thin, 

 segmented hairs, which in F. platycarpus project in a tuft out of the mouth of the 

 conceptacle. The aniheridia are formed on branched hairs as lateral branches of 

 the same ; an antheridium is a thin-walled oval cell, whose protoplasm breaks up into 

 a number of small sperviaiozoids ; these are pointed at one end, and move about 

 by the aid of two cilia ; in their interior is a red spot. The oogonium begins as 

 a papillose protuberance on a cell of the wall of the conceptacle, which is cut off 

 by a transverse septum, and as it elongates divides into two cells, a lower, the stalk- 

 cell, and an upper, the oogonium proper, which enlarges into a spherical or ellipsoid 



Fig. 45. Fitcits i'esiculosj(s. A a branched hair covered with antheridia. B spermatozoids. / an oogonium og after 

 division of its contents into eight parts (oospheres), surrounded by simple hairs /. // oospheres preparing to escape 

 the membrane a has burst, the inner membrane i is ready to open (the two together are an inner shell of the cell-wall of 

 the oogonium). /// nosphere surrounded by spermatozoids. /^'and ^germination of the oospore. B magn. 330 times, 

 the others 160. After Thuret. 



shape and becomes filled with a dark-coloured protoplasm. This protoplasm con- 

 tinues undivided in some genera {Pycnophycus, Hwianthalia, Cystosira,, Hah'drys), 

 the contents of the oogonium forming one oosphere ; in others {Pelvetia) it divides 

 into two, or into four {Ozothallia vulgaris), or into eight {Fucus). Fertilisation 

 takes place outside the conceptacle. The oospheres, enclosed in an inner membrane 

 of the oogonium, are ejected and pass out through the opening of the conceptacle ; 

 the antheridia also are set free and collect in swarms before the opening, if the 

 fertile branches are lying out of the water in a moist air ; if they again come in con- 

 tact with the sea-water the antheridia open and release the spermatozoids ; the 

 oospheres are also set free from their membranous envelope, which is now seen to 

 be composed of two distinct layers (Fig. 45, //). The spermatozoids gather thickly 

 round the oospheres, and fix themselves firmly to them, and when there is a 

 eufticient number of them, their movements are so energetic that they set the large 



