Sphærozyga Carmichaelii. Sphærozyga Thwaitesii.
In some species of this class the continuity of the congregated cells is interrupted, besides by the spore cells, by a connecting cell, or heterocyst, differing in form from either, and not unusually of an entirely opposite and contrasting colour. Such is the case with the Spermosira Harveyana, a very minute species of nostoc, found on dead leaves in the summer month of June. The rudimentary cells of its exquisite curved filaments are small cylinders, the spore capsules completely spherical, and the heterocysts subquadrate, inclining to oval. The colours vary in each, and are in the first of a translucent bluish green,—of course, therefore, the prevailing hue,—which is charmingly relieved by the deep brown of the second and the pale pink of the last.
These constitutional forms, in their varieties and adaptations, their manner of growth and development, constitute the entire structure of the whole tribe of sea-weeds; and therefore we ought to find the chief features of any elegance these humble forms possess continued and elaborated, as they really are, in the more complex conditions of the higher fuci. In the sections of the sea-weeds, therefore, even as made for the scientific elucidation of their structure, we may expect to find, as we undoubtedly shall do, many hints and lessons.
The true form of the cell is perhaps the globe, but it is more commonly presented to us as the cylinder, the conditions and outlines of which are varied almost ad infinitum, as by the various effects of growth and pressure the cells are forced into hexagons, pentagons, and other mathematical shapes, or their lines of junction are disposed in undulating tracery of the most elegant and intricate patterns.
Magnified Transverse Section of Arthrocladia villosa.
Of the few sections we have engraved as illustrations, the first is that of a pretty knotted sea-weed, rather rare, but still not uncommon on the southern coasts of our island in the summer and autumn seasons—the Arthrocladia villosa. Around the tubular axis the larger rings are disposed,—to which circle upon circle of the smaller succeed to the verge of the periphery, yielding to the forms of the intermediate cavities in numerous appropriate shapes. In the second we have given a cross section of the compressed frond of the Desmarestia ligulata, an inhabitant of the tidal pools at extreme low water on most parts of our coasts. An internal jointed tube passes up the centre of the frond, and gives rise to the obscure midrib perceptible on the surfaces of the sides; on either side the larger cells are disposed in two opposing flat arcs, and compressed into shapes more or less hexagonal, outside of which, in the second row, the pentagonal form prevails, and then the intermediate exterior and interior spaces are filled by smaller cellules of more irregular outlines.
Magnified Transverse Section of Frond of Desmarestia ligulata.
