Bauera, also called Native Rose, is, when commonly met with, a pretty little trailing shrub with slender wiry stems, often supporting itself amongst the undergrowth, but under favourable conditions it will spread for many yards, and when luxuriant forms a dense mass of wiry shrub that it is next to impossible to break through. The leaves are comparatively small, and divided into three narrow, equal segments, and as they are placed in opposite pairs, it gives the appearance of having them in circles of six small leaflets arranged at intervals. The flowers are white or pink, and sometimes double, each on a long slender stalk placed singly in the upper axils; they are about half an inch in diameter. There is a plant with yellow flowers and very similar in general appearance, but with simple leaves, which goes by the name of Yellow Bauera. It does not belong to the genus, but is a Hib-bertia. There are generally six or seven small sepals. The same number of rather broader, larger petals, and very numerous stamens inserted on a fleshy disc. The pistil consists of two carpels blended, but with distinct styles and ovaries. The seeds are numerous and without wings. Bauera is the only one of our Saxifrages which is found outside Tasmania, and it extends only to Victoria and New South Wales. Tasmania is exceptionally rich in species of plants that are confined to its area.
We have noticed that in Wattles and Myrtles the foliage is greatly reduced and otherwise modified to suit a climate of excessive sunlight. In the interior of Australia it may be an adaptation to dry soil conditions, but there is no reason to assume that dryness has been the main factor with us. A clear dry atmosphere with considerable temperature is injurious to broad delicate leaves; but in our Saxifrages, though the leaves are not very delicate, they do not show any great effort to reduce exposed surface and evaporation. This mixture of broad-leaved forms suited to mean climatic conditions with others modified to withstand excess is found also through all coastal districts of temperate Australia. We find in these districts that broad-leaved European plants do well; they are not much affected even in a hot, dry summer, provided root moisture does not sink too low. In other words, Tasmania and the coast of Australia have now a climate suited to broad-leaved plants. Why have we then a preponderance of reduced, thick-leaved types? Are they the survival of a former condition, or are they migrants whose hardy nature has enabled them to oust more efficient but less vigorous species ? The migration of plants is a most interesting study, but conclusions should not be formed hastily.
A plant, other than one only suited to live in water, is proivded with a skin which is somewhat analogous to the skin of an animal. Its two principal functions are to protect the soft parts from injury, and to reduce indiscriminate evaporation. Except in some lowly plants, as Mosses, death will ensue if dried by excessive evaporation. The skin effectively prevents this, but as a thick skin brings special disadvantages with it, we generally find species of plants whose natural habitat is a moist, shaded place, or a country with a persistently cloudy sky, have thin skins. Even the same plant will often respond to these conditions, developing a thinner skin if grown in shade than when freely exposed. Though the skin is never absolutely impervious, it is nearly so. In a plant well suited to its surroundings practically no evaporation takes place from the surface; but a plant, in order to live, must draw in through its roots a great quantity of water with dissolved material. And it must get rid of this water by evaporation from its green surface, or the absorption will soon come to a standstill. This evaporation is carried out by innumerable minute pores on the surface of the leaves and green shoots specially constructed for the purpose. These pores are so small that they require a strong lens to show them, yet they are wonderfully constructed. They are not permanent openings, but each has a pair of lips which are capable of opening or closing just as the lips of a mouth. When exposed to light the lips open and the moisture within the tissues can freely evaporate through; when the light fails the lips close together and stop the process. These little pores, from their peculiar structure and function, have received the name of stomata, which means mouths. Each is a stoma. A stoma has another function equally as important to a plant as allowing the escape of moisture, namely, to allow the passage inwards of air. If you take a plant and thoroughly dry it its weight is reduced very considerably. Now, if it is baked at a considerable heat, but not burnt, it. will turn black, or, as we may say, turns into charcoal. Chemists call charcoal carbon. By far the greater portion of the dry part of a plant is carbon. A plant absorbs practically no carbon through the roots; it is all procured from the atmosphere, which enters the green parts through the stomata. As there are on the average only four parts of a gas containing carbon in ten thousand parts of air, it can well be imagined what a quantity of atmosphere must in a year pass in and out of the stomata of a tree to supply its yearly wants.
Bauera. (Bauera rubioides, Andr.) [See p. 67
The carbon of a plant is first reduced from the carbonic acid of the air and then built up by a complicated process with molecules of water to form complex substances in the green tissue: these first appear to us in the form of starch or sugar. This is afterwards used up by the plant to form its multitudinous compounds, or as fuel, to supply energy for growth. What we want to note here is that this marvellous building up of substance only takes place in the green tissue of plants. As all familiar plants are green, we generally take the colour as a matter of course, but it is worthy of the most grave consideration. The green colour is due to a definite substance, plant-green. This substance has the power, when light is suffiicently intense, of splitting up carbonic acid and forming starch or sugar, from which the higher compounds are then formed. Plant-green is the only known substance that has this power. Wherefore not only all plants, but also animals, depend for their existence upon the action of the green tissue of plants.