110 



The Naturalist. 



isolated or in very small families; whereas four genera forming 

 mucilaginous cell families, include twenty-one species, most of which 

 are common. The same ratio holds good for Europe generally, as 

 shown in Rabenhorst's " Flora Europaea Algarum.'^ This community- 

 forming process is far more general among fresh-water than marine 

 algse, and it is only when the minute species of the latter grow 

 between tides, and so are exposed to the air for a considerable time, 

 that this character is met with, as in Petrocelis cruenta, Peyssonelia 

 Dubyi, certain species of Conferva, Rivularia, &c. This difference of 

 habitat is often very marked in species of the same genus, as in the 

 marine eonfervse, where C. tortuom forms fleecy tufts on rocks, which 

 retain a sufficient quantity of water to prevent dessication during low 

 tide ; while C. melagonium, which never flourishes above low-water line, 

 consists of from one to four rigid filaments. The latter is not common 

 anywhere, while the former has a wide distribution. 



Passing on to the multicellular seaweeds of more complicated 

 organisation and in which the individual plants are comparatively 

 large, we find the same idea of solid masses, or dense spongy tufts, 

 characteristic of plants growing between tides. The first structure is 

 illustrated by LeatJiesia fuberiformis, the latter, which is most general, 

 by such plants as CallitJiamnion polyspermum, and the species of 

 Ceramium ; while plants growing in deep water — or if between tides, 

 in rock pools — are of a thin membranous nature, as Belesseria sanguinea, 

 Viva latismna, species of Nitophyllum, Rliodymenia^ &c., which soon 

 suffer if exposed to the air. Hepaticse form the transition from 

 aquatic to terrestrial vegetation, and it is in this family that we meet 

 with marked modifications of structure, for the purpose of enabling the 

 plants to exist under conditions so far removed from those in which 

 their progenitors flourished. The change is not abrupt. Riccia natans 

 is a floating water-plant ; others, as J ungermanma infiata, grow in peat 

 bogs, or similar places very retentive of moisture. Many have a tufted 

 habit of growth ; in such the most evident structural departure from 

 algse is the differentiation of parts, while in Marcliantia polyviorpha we 

 are introduced to an entirely new and complicated structure — the 

 epidermis — which is waterproof, for the purpose of preventing the dry 

 air, to which the surface of the plant is exposed, absorbing its moisture 

 too rapidly. This additional structure necessitates the presence of at 

 least two others, stomata and roots (at least functionally). This new 

 waterproof arrangement does not at once become universal. Mosses 

 which yet retain the caespitose habit, and usually grow in wet places, 

 have no such protection on the vegetative part, but the fruit, which i» 



