A L GA E PHA EOPHVCEA E. 65 



B. PHAEOPHYCEAE. 



The Phaeophyceae, called also Melanophyceae or Brown Seaweeds, are a group in 

 which the thallus is still more highly differentiated than in the Chlorophyceae, as is shown 

 by the fact, that side by side with forms scarcely visible to the naked eye there occur 

 genera, the dimensions of which are the largest to be found among the Algae and 

 among the Thallophytes generally; Macrocystis, one of the Laminarieae, is said to reach 

 a length of two hundred metres. Besides filamentous conferva-like forms such 

 as Ectocarpus, the group contains others which like Sargassum show a differ- 

 entiation into stem and leaf coinciding in outward appearance perfectly with 

 that found in the higher plants. The sexual process has been examined in only 

 a few genera and species ; but what is known is sufficient to show that there is here 

 the same advance from isogamous to oogamous fertilisation as in the Chlorophyceae. 

 Notwithstanding the differences which present themselves on the one hand in the 

 vegetative structure, on the other in the mode of fertilisation, the Phaeophyceae may be 

 arranged in a series, beginning with Ectocarpus and ending with Fucus. The 

 differences within the ranks of the Phaeophyceae are not so great as those which occur 

 for instance in the Confervaceae and Volvocineae ; their subdivisions therefore are to 

 be regarded not as sharply defined groups, but as types which are for the most part 

 connected with one another by intermediate forms. 



The Phaeophyceae are all marine Algae 1 ; the Alga from the Tegler See, 

 described by A. Braun under the name of Pleurocladia, appears to agree in organi- 

 sation with the Mesogloeae ; but it is still imperfectly known, and moreover has not 

 been found in recent times 2 . The Phaeophyceae have this in common, that besides 

 their chlorophyll they contain a brown colouring matter, phycophaein, which conceals 

 the green colour and is the cause of their peculiar brown tint, as phycocyan produces 

 the verdigris-green tint of the Cyanophyceae. The structure also of their swarm-spores, 

 which Thuret's researches made known to us, is characteristic of the Phaeophyceae ; 

 the cilia are inserted on the side of the colourless beak of the swarm-spore, not 

 as in the Chlorophyceae on its point. In the lowest forms the sexual cells 

 are uniform motile gametes (Ectocarpus}, and these may germinate without conju- 

 gation. In Cutleria the gametes are of very different size, the male much smaller 

 than the female, and the latter, soon losing its cilia, becomes a quiescent oosphere 

 with which the male gamete coalesces. In Fucus the difference is still more 

 striking ; the male gametes are here small spermatozoids, and the female cell has 

 lost its power of motion and is ejected from the oogonium, as an oosphere without 

 cilia ; fertilisation therefore takes place outside the plant which produces the sexual 

 organs. Fertilisation of the oosphere in the oogonium, which is so common in the 

 Chlorophyceae and is the universal rule in the Archegoniatae, is not found among the/ 

 Phaeophyceae. 



Putting aside the Tilopterideae, whose development is unknown, and the Dictyo- 



1 [See Flahault, Sur une Algue Pheosporee d'eau douce (Comptes Rendus, 1884).] 



2 As Professor Magnus kindly informs me. 



F 



