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.
[Illustration: _Magnified Transverse Section of Frond of Desmarestia ligulata._]
[Illustration: _Magnified Transverse Section of Spore-bearing Receptacle of Fucus vesiculosus._]