This family is a large and important one, not only from the interest it bears for the student, but on account of the beautiful flowers and useful fruits produced by some of its members. Its natural home is the Northern Hemisphere, where it has developed numerous forms of great variety. In Australia it is very poorly represented with native forms, but in addition to these we have amongst our wild flowers some introduced plants that have made themselves quite at home. In this chapter it will be well to examine some of our cultivated plants as well.
The Rose family is one of the most difficult for the young student to master. It contains shrubs and herbs, but none large enough to be called trees. The habit of the plants, the details of their flowers, and structure of the fruits are so various that there appears to be no one feature we can seize hold of as a mark. Yet the family is natural; that is, there appears an evident likeness in character amongst its members by which we recognise their distinction from all other families. This is one of the great troubles in endeavouring to learn the classification of flowering plants - we have to depend so much on judgment and so little on definition.
There are three principal features we may note as common. The carpels are in nearly every case one-seeded and free from one another. The stamens are numerous, and together with the five free petals are inserted on a cup-like expansion of the thalamus, and not close under the pistil, as in Buttercup. This expansion is small in Plum, but very large in Rose. We shall note its extraordinary development in the fruit of Apple.
We have no native Rose; those flowers which look very rose-like we shall find belong to the Saxifrage and adjoining groups. But Sweet Briar is quite as wild as our farmers care for. We will examine the structure of its flower. The base of the flower is a round or oblong hollow body. In older works it was the custom to call this and all similar developments in other flowers the oalyx tube. In the present day we consider it is not part of the calyx, but an expansion of the flower-stalk, and is called the floral tube. In some few instances there may be doubt on this point, but as a rule when there is a tubular or cup-like expansion it is wise to consider the calyx to commence at the spot where the petals and stamens are inserted. Well, a Rose has a very large round or oblong floral tube, from the top of which come off first a row of five septals, then one of five petals, and close above these a circle of many stamens. In cultivated Roses the petals are greatly increased in number, and this generally occurs by stamens being converted into petals. This change in appearance is not as marvellous as it seems, as all these organs, except the spore-bearing sacks sometimes found on them, as the anthers on the stamens and ovules on the carpels, are simply modified leaves, and may with little difficulty revert to a more leafy condition. We often meet with flowers in which the inner organs revert to the condition of ordinary green leaves, and then the centre generally grows up into a leafy shoot. If you cut a Rose open you will find inside numerous seed-like bodies. Each of these is a carpel, and contains one ovule, which will become the true seed. The carpels each have a long slender style with a little round stigma at the end. These stigmas may be seen protruding in the centre of the flower. When fruit is formed the floral tube becomes fleshy, and red to black in colour. The carpels do not burst to allow the single seeds to escape, but harden and function as a seed-coat. This condition of fruit is useful to the plant in furthering the distribution of its seed. Browsing animals eat these fruits, called Hips, and the seed-containing bodies being quite indigestible, provided they escape being crushed in mastication, are accordingly dispersed.
Cherry, Apricot, Almond, and such have flowers of a much simpler development. The floral tube is small and cup-like, but the sepals, petals, and stamens, like Rose, are placed in succession on its edge. The pistil consists of a single carpel placed in the centre of the flower containing normally a single ovule. In developing into fruit the outer part of the flower withers, and till it falls off remains below, while the fruit develops into a fleshy globe containing a single stone. The wall of the stone as well as the flesh is developed from the wall of the carpel. The kernel is the seed. In Almond the outer coat is green and only slightly succulent.
In Apple and Pear there is a different modification of the Rose type. The floral tube is well developed, but not as much so as in Rose. The sepals, petals, and stamens are similarly placed. The pistil consists of a circle of about five carpels, whose bases are more or less sunk in the substance of the tube. When fruit forms the tube grows enormously, carrying up the withered sepals on its apex, and burying the carpels in its centre. If you cut an apple through, the outer part of its flesh was formed from the tube, and the inner portion and the hard part of the core from the carpels, in each of which are one or two pips or seeds.
We have two native Raspberries, and a very much run wild Bramble or Blackberry. They develop another form of fruit. The floral tube is small, but the centre of the thalamus grows into a cone, upon which are placed few or many carpels. The fruit is formed by each carpel growing into a small fleshy globe with a hard centre containing a seed. The cone of the thalamus grows to accommodate the enlarged carpels.
The Strawberry is a further modification of this. The central portion of the thalamus enlarges into a great fleshy fruit carrying the little dry carpels on its surface
It is unusual for a family of plants to have so much variation in the structure of its fruit as we find in the Rose family, but we must be always prepared to meet great diversity; not only that, but we must not be surprised if we find such a fruit as a berry in many different families. We find the immediate organs of reproduction are altered only slightly in long periods of time. Their position on the plant may be subject to modification, but their characters are ever the same. There is little difference in the pollen sacks of the anthers and the ovum sacks of the ovules throughout the whole range of flowering plants. And they do not materially differ from the same organs found in lower plants. Their function is definite and their character fixed. With fruits it is a very different matter. Their function is to protect and disseminate the seeds in the most effective manner. We find some fruits so hardened that neither animals nor fire can damage the seed; others open elastically, and cast the seed far away; others again are winged, to ensure being blown to new places; while yet others tempt animals to eat them and cast the undigested seed in new situations. Any advantageous change is of great benefit, as it enables a plant to propagate more effectively than its fellows. Wherefore if a plant develops a small but effective improvement, and such sudden developments take place oftener than is generally supposed, it will have an advantage which will enable it to spread more effectively than the others of its kind, which it will soon overwhelm in the struggle for existence. Such a sudden change, if of conspicuous advantage, will therefore become fixed; if not of so much importance, it will generally be obliterated by being bred out by the average.
All plants vary in at least minute details from their ancestors, but cross-fertilisation tends to keep them about the mean. Fruits of an advantageous type are of such great importance that we can readily understand their assuming many forms within the scope of one family. And as their possibilities are limited, it is also easy to understand how the same type may be developed in many parts of the vegetable kingdom. These sudden changes in organisms are responsible for the enormous variety of plants and animals in cultivation. A man does not cause the variation, though he may assist; but when a difference suddenly presents itself he enables it to be maintained and continued by preventing it being bred out by the average or common form.
It is rather interesting to note that throughout the Roses the colour blue appears absent from its flowers. You find red, yellow, white, alone or variously mingled, but never blue. You find a somewhat similar condition in our native Heath. While in Asters the conspicuous flowers may be any shade of blue, red, white, but never yellow. The use of colour is for the purpose of attracting insects, and blue and red colours appear to attract principally in day time; yellow and white are more conspicuous in the dull light of evening and night. There is one class of naturalists who love to see an accurate purpose in every detail of nature. When this is carried to excess it tends first to deceive, and then to disappoint the young student, and neither of these conditions are to his benefit. There are in nature innumerable marvellous adaptations, but it is going altogether too far to claim that every modification we find in a plant has some adaptive advantage. On the contrary, probably in every plant there are many minor structures and qualities that are there as purely negative conditions. They are of no distinct use, nor would their suppression materially benefit the plant. We should be more correct in saying that though a plant must be adapted to its surroundings, yet none is exactly adapted. If it were so, no variation could be to its advantage, and we know variation is the rule, not the exception. Heath and Roses set seed equally well whatever tint their petals may be.
Another class of conditions which are mostly negative, though often of use, are the hairy structures on the skins of plants. Few plants are perfectly hairless. In Roses we find a peculiar development of the hairs in prickles. These are of obvious advantage, sometimes as protective organs sometimes as in the case of the Bramble also to help to support it amongst undergrowth. At other times a copious hairiness protects a surface from being wetted, from frost. or from rapid evaporation. But the slight hairiness so commonly met with has no useful purpose. It sometimes means the survival of a previous more hirsute condition; at others a fortuitous development that may become of use.