Fig. 253.—Delesseria hypoglossum
Among these we may specially mention D. alata, known popularly as the Winged Delesseria, with a dark-red, forked frond, consisting of a strong midrib, bordered by a wing-like lamina of very variable width, supported by opposite veins. In this species the clusters of tetraspores are arranged on each side of the midrib or special leaflets near the tips of the frond. D. sinuosa is a less common weed, with a disc-like root and an oblong, cleft and toothed frond, and tetraspores in leaflets growing from its margin. Another species—D. hypoglossum—is characterised by the leaflets of the midrib bearing still smaller leaflets in the same manner.
We have already referred ([p. 366]) to a sea weed commonly known as the Dock-leaved Delesseria, the scientific name of which is Maugeria (Delesseria) sanguinea. This plant was once included in the present order, but has been removed on account of the different structure of its fruit.
Our next order is the interesting one containing the coral-like weeds, some of which are so common and so conspicuous in the rock pools. The order is known as the Corallinaceæ, and all its species secrete carbonate of lime, which hides their vegetable structure and gives them more the appearance of stony corals.
The typical genus (Corallina) includes two weeds with jointed pinnate fronds, and spore-conceptacles at the tips of the branches with a terminal pore.
These and the allied sea weeds are very unlike plants in their general nature, their stony covering of carbonate of lime hiding all traces of the delicate cellular structure so characteristic of the various forms of vegetable life, and especially those of aquatic or marine habit. If, however, the weed is put into dilute hydrochloric (muriatic) acid the calcareous matter will be completely dissolved in a minute or two, with evolution of bubbles of carbonic acid gas; and if a portion of the frond be then examined in a drop of water under the microscope, the cellular structure referred to will be seen as well as in any other weed. Another characteristic of the plant, or rather of the carbonate of lime which it secretes, is its property of becoming intensely luminous when held in a very hot flame. Thus if a tuft of coralline be held in the flame of a Bunsen burner, it will glow so brilliantly as to remind us of the lime light. Further, if we examine the plant in its natural state, we find that the carbonate of lime is not secreted uniformly in all parts, but that the nodes of the jointed frond are free from the stony deposit, and are therefore flexible.
Our commonest species—C. officinalis—may be found in almost every rock pool between the tide-marks, growing on rocks, shells, and other weeds. The joints of the stem and branches are cylindrical or somewhat wedge-shaped, while those of the branchlets are linear; and the colour varies from a dark purple to white, the former prevailing in the deep and shady pools and the depth of tint decreasing according to the amount of exposure to the bleaching action of the sun.
A second species (C. squamata) is very similar in growth and habit, but is much less common, and is confined to the neighbourhood of low-water mark. It may be distinguished from the last by the form of the segments, which are short and globose in the lower portions of the stem, and become broader and more flattened towards the tips of the branches.