666 THE SUBDIVISIONS OF THE VEGETABLE KINGDOM. 



Alliance XII. — Dietyotales. 



Family: Dictyotaceoe. 



A small group of Brown Sea-weeds distinguished by the fact that both egg-cells 

 and spermatozoids are destitute of cilia. The sexual cells are contained in club- 

 shaped vesicles, inserted in tufts on the surface of the plants. Asexual reproductive 

 cells ("tetraspores") are formed in sporangia in fours. They include the common 

 Dictyota dichotoma and the beautiful iridescent fan-like Padina pavonia. 



Alliance XIII. — Floridese, Red Sea-weeds. 



Aquatic, for the most part marine, plants, which contain in addition to chlorophyll 

 a red or purple pigment, phyco-erythrin; the pigment, as in the brown sea- weeds, 

 is confined to definite corpuscles. Reproduction is by means of asexual spores 

 (teti-aspores), and sexually by non-ciliated spermatia and procarps. 



With the exception of Batrachospermum, Lemanea, and one or two other 

 genera, the Floridese are marine organisms and inhabit on the whole a deeper zone 

 than any other sea- weeds. Several views prevail as to the significance of the red 

 pigment. As has been already indicfited (vol. i. p. 390) the rays of light, useful in 

 ordinary chlorophyll-assimilation, are soon absorbed, as white light traverses con- 

 siderable strata of water. Such light as penetrates some distance from the surface 

 is preponderatingly blue, and, as is now known, such rays are actually destructive 

 of vegetable protoplasm. It may well be then that the red pigment serves to screen 

 the protoplasm from the action of these rays, permitting the chlorophyll to make 

 use of such of the red rays as filter to it; or — what is more probable — the red 

 pigment is itself an assimilating pigment, either directly absorbing the blue rays 

 and allowing the protoplasm of the chlorophyll-corpuscles to use their energy for 

 building up complex food-materials, or indirectly (as indicated at vol. i. p. 390) by 

 altering their wave-length they are made serviceable to the chlorophyll-corpuscles. 



The Floridese exhibit an enormous variety of form, and almost all of them are 

 attached. There are the delicate cell-filaments of the Callithamnions, the corticated 

 Polysvphonias and Geramiums so common on our coasts, the fleshy cylindrical 

 Gracilarias and Pdlyides, the flat and lobed Ohondrus and Gigartina, the leathery 

 Iridcea, and a host of others. One of the most beautiful of British genera is 

 Delesseria, with its creeping stalk and crimson leaves with midribs and veins. In 

 some species the leaves are entire, in others their margins are sinuous and lobed. 

 Of all red sea- weeds perhaps the Australian Clavdea holds the palm for beauty with 

 its large latticed, rose -pink fronds. Certain groups, Corallina, Melobesia, Litho- 

 thamnion, &c., are encrusted with large amounts of carbonate of lime, and build 

 regular banks and reefs under the sea. In all there are some 280 genera and 1800 

 species of Floridese. 



Reproduction by means of asexual spores is a common phenomenon in the group. 

 These spores, though not invariably, are most frequently formed in clusters of four. 



