Alga

“ Evidently the oxygen thrown off by the alga is precisely what the plant needs, and the carbonic acid and nitrogenous waste eliminated by the animal is most useful to the alga. Moreover, the algae gain the advantage of ready locomotion with their host, and the animal can go further into unfavorable media when stocked with algae ready to build up starches and sugary matters from carbonic acid and water. ”

“ This is the case in the remarkable symbiosis between algæ of the family Nostocaceæ and the floating, moss-like water-fern Azolla. This plant, in external appearance almost like duckweed, has on the under surface of its leaves a minute opening, leading into a relatively roomy hair-lined cavity, and in this cavity there is always, enclosed in jelly, a bluish green unicellular alga, Anabæna. The cavity is present in every leaf, and the alga is present in every cavity, making its way in from a deposit of alga-cells which is found on the incurved tip of every young shoot. ”

“ When resting they roll up into a ball, and as a species of green alga not infrequently grows on the fur, they are very inconspicuous among the leaves and moss-covered branches of their home—at least when viewed from below. ”

“ Rich brown alga hangs from its edges in frills and flounces. Little cones stud its surface, each of which is the home of a living, star-like flower, a flower which has the power of displaying and withdrawing itself, and of waving its fringed rays. Each flower is self-coloured, and may represent a group of animals. ”

“ He was able also to prove that this gas came from the associated algæ—specimens destitute of algæ giving off no oxygen whatever.
It is, therefore, now very certain that the yellow cells of radiolarians and the pigment-bodies of cœlenterates are in many cases true alga; living in the animal substance. ”

“ Their principal service is to carry water from the roots to the leaves. From this we can understand how organs essential to the life of land plants can be dispensed with in water plants. They do not need a special conducting of water, because they are surrounded by that element on every side. The most marked instance of the absence of internal organs is met in an alga which forms green fields in the deeper parts of the Mediterranean Sea. It has slender, branching, horizontally creeping stems which develop above in the water into leaves and below in the sand into fine thread roots. ”

“ It is certain, however, that the droppings of the rotge will so enrich the water with nitrates that the red alga, which is growing plentifully, though invisibly, on the black rocks and ground, thrives exceedingly and is carried, and lives and grows on the melting surface of the snow or glacier ice. ”

“ On dissolving the carbonate of lime of shells perforated by this alga, the latter is isolated and appears to consist of rows of small cells, with possibly some sporangia containing spores. Other boring algae have been recorded among the Chlorophyceae, and recently a member of the Rhodophyceae [194] has been found living in the substance of calcareous shells. Such examples are worthy of note in view of the not infrequent occurrence of fossil corals, shells and fish-scales, which have evidently been bored by an organism resembling in form and manner of occurrence these recent algal borers. ”

“ Around the coast-line of land areas, and on the floor of shallow seas algae exhibit a remarkable wealth of form and luxuriance of growth. As regards habit and structure, there is every gradation from algae in which the whole individual consists of a thin-walled unseptate vesicle, to those in which the thallus attains a length unsurpassed by any other plant, and of which the anatomical features clearly express a well-marked physiological division of labour such as occurs in the highest plants. ”

“ In considering the alga-flora of a soil it is necessary to distinguish between two very different sets of conditions under which the organisms may be growing. In the first place, they may grow on the surface of the soil, being subjected directly to insolation, rain, the deposition of dew, the drying action of wind, relatively quick changes of temperature and other effects of climate. ”

“ The substance of the plant is not so solid as that of the common Alga, which is capable of drying as it fades, and of being kept: whereas the leaves of this great Alga shrink and wither in the air, become of a blackish colour, and very friable, or indeed soon fall into putrifaction. ”

“ A green plant can take raw material like earth, air and water and, with the aid of sunlight, make its own food. Algae are found in oceans, rivers, lakes, ponds, marshes and warm springs, everywhere. Some of them are very beautiful, in a great variety of forms and colors. The very commonest algae that you can all find, almost everywhere, is the green scum that forms on quiet ponds and swamps. Scum is all broken up into single plants, or knots andstrings of cells, and can be easily separated and studied under a microscope. ”

“ The relation of the fungus to the alga, though it may be described in general terms as one of symbiosis, partakes also somewhat of the nature of parasitism. The algal cells are usually controlled in their growth by the hyphae and are prevented from forming zoospores, and in some cases, as already described, the algal cells are killed sooner or later by the fungus. ”

“ Then, and still more briefly, have we stopped to see those less known plants that produce no flowers, such as the ferns, mosses, fungi, and finally the seaweeds or algæ. These are all to be considered as the ancestors of flowering plants, the ferns the nearest to them and the algæ probably the most distant relatives. The development of plants from the minutest alga up to our most gorgeous flowering plant, is an infinitely slow and painful process. ”

“ The great majority of the aquatic algae, both freshwater and marine, are attached plants. Some, however, are wanderers, either swimming actively with the aid of cilia, or floating inertly as the result of a specific weight closely approaching that of the medium. ”

“ The frog-spawn alga, therefore, will not thrive in any but the purest water, and a gentle flow is necessary to its growth and health. "These plants are so exceedingly flexible," Dr. Hassall continues, "that they obey the slightest motion of the fluid which surrounds them, and would seem almost to be endowed with vitality; nothing can surpass the ease and grace of their movements. When removed from the water they lose all form, and appear like pieces of jelly without trace of organization. ”

“ In the case of the freshwater algae, however, belonging to the Chlorophyceae and Cyanophyceae, although they required to be immersed during the vegetative period, the reproductive cells are often capable of resisting a considerable Habitat.degree of desiccation, and in this condition are dispersed through great distances by various agencies. Again, as is well known, many species of marine algae growing in the region between the limits of high and low water are so constituted that they are exposed to the air twice a day without injury. ”

“ Several genera live as endophytes in the tissues of other algae. The recent species of this section of algae are characteristic of temperate and tropical seas. One subdivision of the red algae, the Corallinaceae, is extremely important from a geological point of view and must be dealt with in some detail. ”

“ On this physiological basis “soil-algæ” may be defined as those micro-organisms of the soil which have the power, [100] under suitable conditions, to produce chlorophyll. Such a definition has the advantage that it is wide enough to include the filamentous protonema of mosses, which, though alga-like in form and in physiological action, is nevertheless separated from the true algæ by a wide gulf. A more accurate name for such a group of organisms would be the “chlorophyll-bearing protophyta” of the soil ”

“ The number of species of vegetal organisms below this depth is extremely small. An alga (Halosphæria viridis) has been brought up from depths of one thousand to two thousand metres.
A very great number of bacteria are unfavorably affected by light, and find proper conditions at some depth in the soil or water. It is on account of this fact that the water of frozen streams becomes more thickly inhabited by certain organisms than in the summer time, and exposure to sunlight is adopted as a hygienic measure in freeing clothing and household effects from infection. ”

“ The bed of the ocean, from the tiny lakelets left by the retiring tide to the greatest depths ever reached by trawl and dredge, is found to be teeming with exquisite forms of life. Delicate plant-like forms are found clinging to rocks and shells, or spreading themselves over the broad fronds of the algae. Every peculiarity of vegetation is mimicked; graceful stems rising from tangled roots send out branches which bear raceme-like clusters of buds, and delicate bells whose beauty no words can describe.
Fig. 5.—Roving Medusa of Tubularia indivisa. ”

“ There is considerable difficulty in removing mounted specimens of algae from paper, and therefore a small portion preserved on mica should accompany each specimen, enclosed for safety in a small envelope fastened at one corner of the sheet of paper. Filamentous diatoms may be mounted like ordinary sea-weeds, and, as well as all parasitic algae, should whenever possible be allowed to remain attached to a portion of the alga on which they grow, some species being almost always found parasitical on particular plants. ”

“ Each class of flowerless or cryptogamic plants requires special treatment for the herbarium.
Marine algae are usually mounted on tough smooth white cartridge paper in the following manner. Growing specimens of good colour and in fruit are if possible selected, and cleansed as much as practicable from adhering foreign particles, either in the sea or a rocky pool. Some species rapidly change colour, and cause the decay of any others with which they come in contact. ”

“ The evolution of the algae has led for the most part to the development and long continuance of such phases of the life history as are stationary, and from these the filamentous, membranous and otherwise differentiated plant bodies have arisen. ”

“ The comprehensive generic name Algites [245] has been suggested as a convenient designation for impressions or casts which are probably those of algae.
SUPPOSED FOSSIL ALGAE.
Some of the fossils described by Mr Kidston from British Carboniferous rocks as probably algae present an undoubted algal appearance, and might be placed in the genus Algites; but in some cases—e.g. Chondrites plumosa [246] Kidst. from the Calciferous Sandstone of Eskdale, one feels much more doubtful; in this particular instance the impressions suggest the fine roots of a water-plant. ”

“ These algae are a genuine prodigy of creation, one of the wonders of world flora. This family produces both the biggest and smallest vegetables in the world. Because, just as 40,000 near–invisible buds have been counted in one five–square–millimeter space, so also have fucus plants been gathered that were over 500 meters long!
We had been gone from the Nautilus for about an hour and a half. It was almost noon. I spotted this fact in the perpendicularity of the sun's rays, which were no longer refracted. ”

“ Lastly, there are blue-green Algae, jelly-like masses found on trees, rocks, damp earth or floating as green slimes in fresh water. Most water plants are active and independent. They are on the upward road, for though they have not distinct stems, roots, leaves or fruit, their different parts, as I have already said, show a decided likeness to these, especially their “holdfasts” to roots and their air-bladders to fruit. The exquisite red seaweeds are as graceful in form and vivid in color as many flowers. ”

“ There is left for us a numerous and varied array of algae, representing several lines of ascent, all tending to diverge from one another. But these forms have some important points in common, particularly as concerns certain events in their life histories. There is immense variety in the form of the plant body which ranges from a single cell to structures with stalks and leaf-like organs. ”

“ The rapidity with which walls, piles and pontoons—stone, wood and iron—become covered with marine plants is well known, while the discovery of some effective means of preventing the fouling of the bottoms of ships by the growth of algae would be hailed as a boon by shipowners. While rocks and boulders are the favoured situation for the growth of marine algae, those which readily disintegrate, like the coarser sandstones, are naturally less favoured than the hard and resistant. A large number of algae again live as epiphytes or endophytes. ”

“ Many of the fresh-water algae which form a mere crust, such as Palmella cruenta, may be placed in a vessel of water, where after a time they float like a scum, the earthy matter settling down to the bottom, and may then be mounted by slipping a piece of mica under them and allowing it to dry. ”

“ Before discussing our knowledge, or rather lack of knowledge, of fossil algae at greater length, it will be well to briefly consider the manner of occurrence and botanical nature of existing forms. In the sea and in fresh water, as well as in damp places and even in situations subject to periods of drought, algae occur in abundance in all parts of the world. ”