8 MELANOSPERME.E. 



intended for vegetation in shallow water is further proved by 

 the air-vessels with which most of them are furnished, and 

 which enable them to keep their long but flaccid fronds in an 

 erect position, with the uppermost branches either floating on 

 the surface of the water, or submerged but a short distance 

 beneath. In specimens growing in shallow water near 

 high-water mark the air-vessels are either absent or in small 

 quantity, but in those that grow at a lower level they are 

 proportionally abundant. And in the Sargassum bacciferum, 

 the famous Gulf-weed, which floats on the surface of the 

 great ocean, the air-vessels are in such abundance as to form 

 the most striking feature of the species. 



Some of the Melanospermeae are of great size, by much 

 the largest of known Algae, surpassing in the length of their 

 fronds the tallest forest tree ; but comparatively few of them 

 attain such a proportionate diameter in their stems as to en- 

 title them to be called arborescent. In the deep bays of the 

 southern hemisphere, along the shores of the Falklands and 

 among the Archipelago of Cape Horn, the species of Lesso- 

 nia and Durvill&a do indeed resemble submarine trees, with 

 gigantic leaves pendant from the tips of robust branches : 

 and even on our own shores the fronds of the larger tangles 

 (Laminaria), seen through clear water of one or two fathoms 1 

 depth, have a similar character, and enable us to conceive 

 what glorious objects their greater southern analogues must 

 be when thus seen, waving freely below us. All the larger 

 kinds grow on rocks, to which they are firmly attached by a 

 root or holdfast, which is almost always conical, and which 

 adheres with great force to the rock. In many the cone is 

 solid, a compact mass of tough cellular tissue, but in others, 

 as in most of the Laminarice, the cone is composed of nu- 

 merous stout, branching fibres, growing out, like the aerial 

 roots of the Screw Pine, one above another, and each with 

 its extremity taking fast hold of the ground ; so that, with 

 the increasing growth of the frond, the base is proportionably 

 strengthened. Some few, like Pycnophycus, spring from 

 prostrate or creeping stems, which form a matted network 

 over wide spaces of rocks, and throw up at intervals erect 

 fronds, that then appear to be densely crowded together. A 

 great many of the smaller kinds are parasitic, or at least epi- 

 phytic, attached to other Algae by minute disks, in every 

 respect, except size, similar to the conical bases of larger 

 species. Some are true parasites, as the Elachistea and 

 Myrionemata, which seem to be incapable of independent 



