Some of the foci are quite cartilaginous in texture ; such as the stem and midrib of F. esculentus, and the whole plant of F. aculeatus. A few species dissolve into jelly on being placed in boiling hot water. Some are quite flexible when recent, and become stiff and apt to break when dry ': others, on the contrary, as the genus Dictyopteris of La mouroux, are stiff and frangible when living, and become flexible when dried.
The jointed structure of the frond was long considered as characteristic of Confervce ; but this, it is now admitted, does not effectually distinguish them from several fuci, such as F. frmiculosus and pinastroides, or even some li chens and fungi; and it has therefore been abandoned as a generic distinction. Lamouroux remarks, that it is easy to distinguish the truly articulated sea-weeds, or marine confervx, from those fuci that are marked by contractions or partitions. If the stem of these last be cut longitudinally, the substance spews no mark of interruption, but appears quite homogeneous. In the former, the tissue appears at every joint to change its quality and form, and the colour varies. Some of the small cylindrical foci seem to acquire the appearance of contractions or partitions just before the time of fructification; nature perhaps producing these par titions, to give firmness to the stems, or to delay the cir culation of the juices,and make them undergo a more com plete elaboration at this crisis. It was first observed by Mr Dillwyn, that the aculei of F. aculeatits are regularly jointed, and that the main filaments towards their extremi ties have a jointed internal tube: all of these jointed ap pearances vanish as the plant grows old. The tufted fibres of F. radiciformis (Turn. t. 189.) are also jointed; as are those likewise of (t. 188). F. dasyphyllus (t. 22,) it may he noticed, is sornetimes, but rarely, jointed; generally it is without joints.
No pact of the structure of foci has more universally attracted notice than the iodated portions of the stem or frond resembling bladders, seen in many species, and par ticularly conspicuous in the well known F. vesiculosus and nodosus. (See Plate CCI,XI. Fig. 4, 5.) They are now generally denominated ,1ir-vesicles, or simply Vesicles. Most of the sca•weeds having a texture approaching to ligneous, arc provided with distinct air-vesicles ; and those that arc not so provided, have in their stems empty spaces, sometimes visible to the naked eye. These probably serve
the purpose of air-vessels : they proceed from the root or base, rise with the stem, and disappear in the frond.
I'm meth', the air-vesicles were generally supposed to be the male fructification. The generic character of Focus, given by Linnaeus, was the following: "Male, Vesicles smooth, hollow, with villosc hairs within, interwoven. Fe male Vesicles smooth, filled with jelly, sprinkled with im mersed grains, prominent at the tip, seeds solitary." If the reader imagine to himself the application of this cha racter to the common F. nodosus, he will perceive, that by male vesicles, must be meant the air-bladders; and by fe male vesicles, the receptacles, the immersed grains PrOnli• rent at the tip, being a sufficiently distinct description of the tubercles. They were afterwards for a long time con sidered as destined merely to give buoyancy to the plant. It is now, however, generally believed, that they serve some important purpose in the economy of sea-weeds. Lamouroux considers them as respiratory organs ; and he observes, that where they arc wanting, the organization is less complicated, and the vital functions ate more simple. Ile affirms, that sea plants exert the same action on atmos pheric air as land plants. The ligneous fucacean he says, absorb oxygen during the night, and give it out during the day, though in very small quantity. Analogy would lead us to consider this statement as probably correct ; but whether it is founded on direct experiment, is not explain ed by the author. The Noridex, he adds, exhale less oxygen than the former. They retain it, in his opinion, in order to produce the brilliant tints which adorn them. The tilvace,n, like hel'iaceous land plants, give out by the ac tion of light, a great quantity of oxygen gas, and a little carbonic acid. The decomposition of the atmospheric air must take place in the vesicles, or in the spaces or cells already alluded to ; these perhaps at once serving as reser voirs, and as organs for decomposing the air. On some species, vesicles have never yet been found; but Turner is inclined to think, that none are entirely and at all times destitute of them ; so that the division'' absque vesiculis" Might be given up.