A typical plant, is green in colour, and it is so because it possesses a quantity of a peculiar substance called plant-green, or chlorophyll. This substance contains a minute quantity of magnesium. By means of the energy derived from light, this has the power of decomposing carbonic acid gas, and, after a complicated process, of producing starch or sugar. After this, by further combination with salts received by means of the water absorbed by the roots, all the multifarious compounds required as food by the plant are well built up.
All plants have not this plant-green, and therefore do not perform these acts. The fungi, forty thousand strong in species, are without it; so are isolated cases, even amongst flowering plants. When this is so the habit of a plant is similar to that of an animal, in so far that it is dependent for its well-being on being able to absorb compounds already formed.
There is no rigid line between plants with plant-green and without. There are all grades of intermediate condition where it is present, but not in sufficient quantity to enable the plant to do all its work, where it is partly dependent on its chlorophyll and partly upon absorption.
It is probable that normally green plants cannot absorb high compounds through their roots. Though the mixing of such, as manures, are of great use, they are first reduced to a relatively simple condition before they can be taken up.
The first condition of departure from the normal is for a plant to gain the power of absorbing the material of dead animal and vegetable remains. As it gains this power it ceases to construct chlorophyll, and therefore ceases to be green. It may be white or any other colour but green. Some orchids and the greater number of fungi are in this condition. They thrive amongst rotting plant remains, and assist in the breaking up of such material and bringing it into use again. Plants that live in this manner are called saprophytes.
The next condition we notice is that plants may form an attachment to others, and suck some of their required nutriment from them while still retaining the power of forming food in the ordinary manner. These are partial parasites, and they may attach themselves by their roots underground, or may grow on their branches. Native Cherry and Eyebright and the Australian Mistletoes are examples of this.
Butterfly Plant. (Utricularia dichotoma, Lab.) [See p. 130
Further, plants may become entirely parasitic, as many of the fungi causing diseases of plants. Spotted Orchid and Native Potato are root parasites: our Mistletoe and Dodder are parasites on plant stems.
A very interesting means of adding to the stock of food without the trouble of manufacturing it is by catching and consuming animals. This is the carnivorous habit. No plant has the power of preying on animals larger than insects; all further statements, as plants consuming large animals and even man, must be taken as travellers' tales. There are a good many carnivorous plants in the world, and in Tasmania we have at least ten species belonging to two genera, the Sundew and Butterfly plants.
We have no conspicuously saprophytic flowering plants, except a rare little Thismia. Probably research will discover that more of our wild flowers have this power to a partial extent, but it has yet to be proved.
Of the partial parasites the most conspicuous is the family to which our Native Cherry belongs. It is the Sandalwood family, all of which have this habit. We have seven species, of which Red and White Cherry are the commonly noticed plants. These are very different in habit, and their fruits are not always red and white respectively, but are so sufficiently often to make it an easy way for immediate recognition. These plants have minute functionless leaves, but the branches are green, and do their work. When quite young they live independently, but soon, where their roots meet with those of other plants, they attach themselves and suck nourishment. If you clear all vegetation round a Native Cherry and trench sufficiently deeply to cut all root connections, the shrub will proceed to die; also, all efforts to transplant other than a small specimen will result in failure. The flowers are very small, and arranged many together, close on the ends of branches. The perianth is simple and minute, and placed below the ovary; this distinguishes it from a common closely allied genus, where the perianth is above and crowns the fruit with a little crest. As fruit is formed the stalk of the flower elongates and becomes very fleshy and red or white. The ovary bearing the seed is borne on the apex like a little oblong hard berry. This fleshy stalks induced someone with a strong imagination to call the shrub Cherry, and gave rise to the painful exaggeration that in Australia the cherries bore their stones on the outside. The fleshy stalk is of much use as an aid to dispersal. Native pigeons are very fond of it, and swallow the whole fruit. The ovaries are resistant to digestive juices, and are subsequently distributed far and wide.
Native Potato. (Gastroides sesamoides. R. Br.) [See p 130
Eyebright is a pretty little herb, common from seashore to mountain plateaux. The flowers are numerous, irregularly two-lipped, white or mauve, sometimes striped, and with yellow markings. Though the leaves are fairly developed, the plants always appear to avail themselves of the roots of others to increase their supply of food. There are probably other partial parasites that patient examination will reveal.
Pink-spotted Orchid and Native Potato, both of which have been sufficiently described in the last chapter, have a similar habit. They do not develop plant-green, but in very early life attach themselves to the roots of other plants, at whose expense they grow. For a long time they remain underground, gradually storing up food in rather large tubers. When sufficient is accumulated and conditions are favourable, they proceed to send up into the air their large inflorescences. Though the stem dies after seed is matured, the underground portion lives on for some years till the roots supplying food ceases to be available.
Our Native Mistletoe is very badly named, as it is no relation to either European or Australian Mistletoe; also, it is very unlike either. The structure of the flower shows it to be a true Laurel. This confusion of popular names is not confined to Australia, and is one of the reasons why botanists do not use popular names oftener. Such use would only cause hopeless confusion. Our common Native Laurel has no right to the name. The European Laurel so common in English and Tasmanian gardens also is no Laurel; it is a plum. The imported Baytree is a true Laurel, Laurus nobilis; our Mistletoe is another. Till we can establish a uniform and sensible list of popular names, the gentle public must bear with the pain inflicted by scientific appellations. Well, of these little plants, which we will not call Mistletoe, but by their proper name, Cassytha, we have three species; they are all wiry leafless parasites that form string-like tangles on shrubs. They have no connection with the ground, but wind themselves round the branches of their hosts, and here and there, where they touch, they form little cushions of tissue, in the centre of which a process burrows its way in, and not only forms a means of attachment, but a permanent means of sucking nutritious juices. The largest species is Black Cassytha, which is rather coarse and bears a black berry. It does not occur in the south. Velvety Cassytha is the commonest. Very often found in Buloke; sometimes on other shrubs it forms a dense mass and has a round green berry covered with delicate microscopic hairs. Smooth Cassytha is more slender and spreading. It grows on small shrubs in heathy places, and bears a small oblong reddish berry.
The flowers are minute and few, close together. Each has six small perianth segments in two series of three each; twelve little stamens also in two series. Some of them are not perfect, but where they are the anthers are interesting objects. Instead of opening in the ways of anthers we have already described, they do so by a little valve which opens below and curves upwards. This is a condition found in all Laurels. Dodder, which is such a pest in fields of Lucerne, has a similar habit to Cassytha, but it belongs to quite a different group. The structure of its flower indicates it to be a degenerate member of the Convolvulus family.
The carnivorous habit of some plants is always a marvel to the observer; it seems such a great departure from what we figure to ourselves as proper behaviour. Yet they are much more common than is generally supposed. As they can only capture small beings, they are generally called insectivorous plants, which is not strictly correct, for other animals besides insects often fall a prey to them.
We have about six species of Sundew or Drosera, which are all noticeable for having leaves adorned with many hair-like structures, at the tip of each of which is a dewlike drop of sticky fluid. This contains a ferment very like that contained in the gastric juice of animals, and has the power of dissolving flesh. If a small fly or an ant finds its way on to a Sundew leaf in good health, the hairlike structures bend towards it till their fluids cover its body. This, from its viscidity, impairs movement, and prevents escape. The muscles are now slowly dissolved and absorbed into the tissue of the leaf, the hard skeleton remaining behind. The same act is induced by placing a minute piece of white of egg or meat upon the leaf. It is further singular that these tenacles have the power of discriminating between flesh-containing and other substances. If a useless bit of material is placed upon the leaf they do not respond. Droseras can do without animal food, but thrive better when it is added to their diet. If it is given in excess the health of the plant is injuriously affected.
Droseras have flowers of the Saxifrage type, and are only separated from that family on account of their peculiar habit. There are four or five sepals, the same number of white or pink petals, and generally the same number of stamens; all are inserted into the thalamus close below the pistil.
We have seven species, four of which have small shield -like or kidney-shaped leaves. One of these, Drosera pyg-moea, is very small, bright red, and close to the ground; it could be completely covered by a sixpence. Drosera binata is often a foot high, with leaves shaped like a tuning-fork. Drosera arcturi grows on the top of mountaine, and bears long flat leaves and single white flowers.
The other genus of our carnivorous plants is more scarce, and though there are four species, only Butterfly Plant is likely to be gathered. The flowers are generally borne in a single pair at the apex of a slender stalk a few inches high. Purple or white, with a small upper lip and a large spreading lower one; they are very conspicuous in some wet places. Where they grow on the ground the leaves are few, small, narrow, and green, close at the base of the stalk; but when the leaves find themselves in water, they grow into long branched strings that bear numerous little colourless sacks. Each sack has a peculiar mouth that permits the entrance of a small animal, but prevents its escape. Little glands on the inner surface secrete a fluid similar to gastric juice, which proceeds without apology to digest the unfortunate victim.