THALLOPHYTA— ALGiE 47 



latter group, which are developed, not in sori, but in conoeptaeles 

 much like those of Fuous. 



The CuTLBEiACEffi have a special interest as illustrating the 

 evolution of sex in the Phseophyeese. They possess two kinds of 

 gametes which are both ciliated, the female, generally called 

 oospheres, bemg much larger than the male. The organs in 

 which they arise are situated in sori on the surface of the 

 thallus, and the gametes are set free by a lateral aperture in the 

 waU. When they are liberated both kinds are freely motile ; 

 the female comes soonest to rest, and is fertilised by the fusion 

 of an antherozoid with it. 



The thallus is sometimes erect and sometimes prostrate. It 

 grows by means of a cluster of hairs, the bases of which are 

 merismatio. The rows of cells produced from each hair become 

 united together laterally to form the tissue of the thallus. 



The differentiation of the internal tissue is not so complete 

 as in the other groups described. In Cutleria itself three sys- 

 tems can be seen ; the epidermal layer, which contains chromo- 

 plastids in abundance, a cortical layer, of clearer cells in 

 which a little colouring matter is present, and an axial layer of 

 larger cells, almost colourless. All the cells are parenchymatous, 

 the pseudo- vessels of the former groups not being represented. 



Asexual reproductive structures are met with in the Cut- 

 leriacise, which take the form of unilocular sporangia, each con- 

 taiuing several zoospores. These resemble the gametes in 

 being ciliated, but they are intermediate in size between the 

 antherozoids and the oospheres. In the genus Cutleria these 

 are borne upon the young plant produced from the germinating 

 ■ zygote. The form of this sporophyte differs considerably from 

 the thallus which bears the sexual cells. There is thus in 

 Cutleria an alternation of generations. In the other genus of 

 the group, Zanardinia, the two forms are essentially similar. 



The sexual organs are placed in sori, and originate in hair- 

 like outgrowths of the thaUus. The hairs bearing antheridia 

 are often much branched, as in the case of Fucus. The sori 

 contain paraphyses as well as sexual organs. 



The DiCTYOTACB^ form a group somewhat resembling the 

 Ciitleriaceae, but differ in the gametes being noncUiated. The 

 thaUus is very varied in form, b^it is generally flattened. The 

 branching is dichotomous, and usually arises from a longitudinal 

 division of a smaU-oeUed apical meristem. A single apical cell 

 is only known in the genus Dictyota. There is not much dif- 

 ferentiation of internal tissue, an epidermis being however well 



