ALG.E. 439 



called a tetraspore (fig. 510, c), which appears to be the representative of 

 gernmiparous reproduction, nor in Fucaceae, where the only known kind 

 of reproduction is by sexual organs. 



Sexual reproduction has been made out clearly in Algae belonging 

 to the Confervoid and Fucaceous groups, and in the Rhodospermeae. 

 The Phaeosporeae at present are only known to produce zoospores. The 

 essential phenomenon throughout is the emission from an antheridium 

 of antherozoids which are endowed with a power of locomotion, and 

 ultimately come into contact with a cell, which, in consequence, develops 

 into a spore, which may be one of the ordinary cells of the thallus set 

 apart for this purpose, or may be contained in a special fruit. 



In the Confervoideae, where the spores are developed from ordinary 

 cells, there are no special reproductive organs ; the spores formed in the 

 impregnated cells acquire thick coats (fig. 512, d), usually assume a red or 

 brown colour, and are set free by the decay of the parent cell. 



In the Fucaceae the fructification is limited to definite parts of the 

 thallus. In Fucus or Halidnjs (fig. 51]), which may be taken as ex- 

 amples, the reproductive structures are formed at the ends of the lobes 

 of the thallus. Externally the lobe (called the receptacle) presents a 

 thickened appearance, marked with numerous distinct orifices (fig. 511) ; 

 these orifices lead to chambers imbedded in the thickness of the thallus 

 (called conceptacles, b), bearing on their walls cellular sacs of two kinds 

 one, the larger (spore-sacs, e), containing the spore-germs, the smaller 

 (antheridia, c) containing spermatozoids (d) or impregnating corpuscles ; 

 both kinds of sacs burst and discharge their contents when ripe, and 

 the spores are fertilized and encysted while swimming freely in the 

 water. 



The Rhodosperrneae and Dictyotaceae, besides tetraspores, have spore- 

 sacs and antheridia, mostly collected in " fruits " of definite form, some- 

 times in patches or lines (sori) on the surface of the thallus, like the sori 

 of Ferns, sometimes imbedded in definite groups in its substance (called 

 favellce), sometimes projecting more or less from the surface or margins 

 of the thallus (fig. 510, D, d, E), when they are naked or surrounded by 

 a gelatinous or cup-like involucre (favellidia, coccidia, ceramidia). The 

 antheridia are usually found arranged in groups in similar situations 

 (tig. 510, F, ) ; and the tetraspores are either scattered or collected in 

 fruits analogous to those containing the spores and antheridia (fig. 510, 

 13, c). The antherozoids are immobile, and fertilize the sporange by 

 means of a special tube projecting from the latter and called the tricho- 

 f/yne. Transformed branches containing imbedded tetraspores are called 

 stichidia. The sexual organs are often found on distinct plants, which are 

 thus dioecious. 



The Oscillatoriaceae are at present only known to increase by division 

 that is, vegetatively ; the Phasosporeae, again, are only known to 

 propagate by 'liberation of zoospores from special cells of the thallus. 

 The Dictyotaceae and the Rhodospermeae produce a peculiar kind of 

 vegetative offset called a tetraspore, a body formed mostly in special loca- 

 lities or in groups, and consisting of a parent cell divided' into four cham- 

 bers, the contents of which, when set free from the parent plant, grow 

 up at once into a new thallus. Besides the tetraspores, they have spores 

 and antheridia. The antheridia produce minute, ultimately free vesicles, 

 spermatozoids or antherozoids, according to Thuret devoid of cilia and 



