its holes and crevices, grasping it so firmly as to defy the efforts of the waves to tear the plant from its station. In a few instances the distinction between root, stem and leaf, vanishes altogether, and the plant becomes a mere crust growing over the surface of the rock, or a shapeless mass, either hollow or filled with loose branching fibres. A few of the brown seaweeds are almost microscopic, forming a minute tuft of fibres, scarcely visible to the unaided eye.
The shape and structure of the frond is often very curious. In the common Chorda filum, the frond is a hollow thong, about the thickness of whipcord, divided at intervals into chambers by their partitions. In one common and very curious species—Himanthalia lorea—which we may translate Strapweed, the plant in the first year of its growth has much the same shape as a pegtop fixed to the rock by the peg, the upper surface, however, of the top, being concave or flat. In the second year a long flat strap shoots from the centre of the top, which strap would, by the uninitiated, be certainly mistaken for the frond. The microscope, however, shows that it is not the frond, but the fruit or seed receptacle of the plant. Not the least attractive of our English seaweeds is the pretty little Padina Pavonia, so called from its fancied resemblance to a peacock’s tail. The grey powdery fronds of this plant, with their delicately fringed margins, have a peculiarly exotic appearance; and in fact, Padina Pavonia, though not uncommon on our southern shores, is more at home in warmer seas. With us it shows its love of warmth by choosing shallow rock-pools exposed to the full rays of the summer sun, and by only attaining its full size in our warmest summers. A few of the brown seaweeds are beautifully iridescent, displaying in the water brilliant prismatic colours. The Cystoseira, whose glowing green is only visible in the summer months, we have already mentioned, and Padina Pavonia occasionally affords an instance of the same peculiarity. The flat, fan-shaped, repeatedly-divided frond of Cutleria multifida, frequently glows with a brilliant series of prismatic colours.
But the chief point of interest in the brown seaweeds, as in most flowerless plants, lies in the study of the problems connected with their fructification, and with the various processes by which the species are reproduced. In many of them we find instances of that strange process of reproduction by means of zoospores, or moving spores, which so often occurs among their green-spored relations. The zoospores may be well seen in the plants which belong to the genus Ectocarpus, one or two species of which are very common in the summer, spreading in a matted mass over rocks and larger seaweeds, and looking when left by the tide like a layer of withered and rotting leaves. The moving spores are formed in the last cell of each of the fine threads of which the plant is composed. They are small, brown, pear-shaped bodies, each bearing a minute red spot, from which spring two cilia or vibrating threads, one much longer than the other. The longer of these is directed forwards, and seems to be the chief agent in producing the motion, while the shorter trails behind and acts as a kind of rudder.
But in addition to this multiplication by zoospores, there exists in many, perhaps in all these plants, a yet more marvellous mode of propagation. We will try to describe this process as it occurs in the common Fucus platycarpus, or Flat-fruited Fucus, choosing this plant partly because it is one of the commonest and best known forms, partly because in it the various organs and structures concerned in the process always exist together in the same individual.
If we examine a number of these plants—especially during the winter months—we shall find that in many of them the end of the frond is expanded into a flat swelling, roughened on either side with numerous small nipple-shaped warts. It is in this thickened part of the frond that we must look for the fruit. By examining a thin slice with the microscope, we find that each of these warts is hollowed into a spherical cavity, communicating with the air by means of a small pore. The interior of the cavity is thickly lined with a great number of fine fibres, some of which are very long and project through the pore, forming a small tuft on the exterior. In some of these fibres, lying within the cells of which they are composed, may be seen certain small oval bodies not unlike the zoospores just now described, each having a bright orange dot and two long thread-like cilia. These bodies, like the zoospores, when discharged by the rupture of the cells containing them, have the power of moving rapidly through the water by means of their vibrating cilia. But unlike the zoospores, if left to themselves they soon lose the power of moving and quickly decay. What their true nature is, and what part they play in the life history of the plant, we shall see directly; for the present we will only remark that they are called “antherozoids,” a term for which the English language has no equivalent.
Returning to the slice of the frond which we have been examining, we see nestling among the fibres with which the interior of the cavity is lined, a few rather large, dark, pear-shaped bodies, fastened by their smaller end or stalk to the wall of the cavity. These are the “sporangia,” or seed-vessels. When ripe they burst, discharging eight or in some cases four spherical brown spores. These spores have by themselves no more power of growing into a new plant than the antherozoids; like them, left to themselves, they soon decay. It is only by the union of the two that a new plant can be produced. In the species which we have been describing, this union takes place within the frond, and it is therefore very difficult to get a peep at the process. But in the equally common Bladder-wrack, the antherozoids and spores are always found on distinct plants, and the fructification of the spores takes place after they have been discharged from the frond. Plants containing spores may be recognised by the olive colour of the thickened extremity of the frond, those containing spores by the same portion being orange or yellow. By mixing in a drop of sea-water<!— see list of hyphenated words --> a small portion of the contents of the olive and yellow receptacles, the whole process may, with the aid of a good microscope, be easily seen.
