Flowers And Their Formation
( Originally Published 1915 )
Conditions favouring the production of flowers. Structure and functions of the various parts of a flower. Pollination and Fertilisation. Self-fertile and self-sterile flowers. Ripening of fruits and seeds.
In speaking of the difference between vegetative and reproductive organs, in the first chapter I have already mentioned that certain external conditions which favour the development of the former affect adversely the formation of flowers. Long horticultural practice has also proved that the development of floral organs can be stimulated in various ways which are well known to gardeners. Thus reduction in the supply of water and consequent stoppage of the luxuriant development of leaves is one of the chief methods employed. It has been found by scientific experiments that these various horticultural practices are based upon definite physiological requirements. In recent years much has, been done to confirm and extend our knowledge of this subject. It was proved that bright illumination of plants is essential for the production of flowers. At first it was thought that certain rays of light influenced the development in the plant of special flower-forming substances, and early experiments seemed to indicate that these were produced by definite rays of light from beyond the blue end of the spectrumórays of light which are known to have great chemical activity. More recently, however, it seems to have been established that it is probably the intensity rather than the quality of light which is required for flower production. Another factor which is important in this respect is the concentration of organic material In the stem and leaves of the plant, while an increase of water and inorganic salts tens to the development of foliage rather than flowers. It is for this reason that a reduction in the, supply of water is so helpful in producing abundant flower buds. It is sometimes thought that a wealth of blossom on our fruit trees predicts a fine summer; from what has been said above, it is obvious, however, that it is the fine summer or dry autumn of the previous year which is responsible for the prolific bloom. Flower buds on our fruit trees are, of course, already developed in the late autumn. Dealing with herbaceous plants, it has been found possible to arrest the development of flowers even if the Powering shoot has commenced to make its appearance. Thus in specimens of the common House Leek a rosette of vegetative leaves may be caused to appear on the flower shoot if the plant is copiously watered and illuminated with light passing through a red or blue screen. Conversely, the runners of some plants may be made to bend upwards and develop flowers if water is withheld and the plant placed in a very bright light.
The horticultural practice of pruning and root pruning is also intended to further floral development. Each tree or shrub must be treated differently according as to whether flowers normally make their appearance on long shoots or on spurs, i.e., on new or old wood. The removal of non-flowering shoots is therefore what is aimed at in summer pruning the stoppage of the growth of purely vegetative shoots will actually stimulate the older branches to form flowers, as they will have an increased amount of food material at their disposal. The increased production of flowers effected by root pruning is often remarkable. It will be sometimes observed that apple and other fruit trees produce long and vigorous new shoots, particularly in an upward direction, which are caused by the development of deep roots able to obtain an abundant supply of water even in fairly dry soil. The removal of these deep roots stimulates the growth of fibrous roots in shallow soil, where there is a less abundant supply of water in the summer and early autumn ; the result of root pruning is therefore the production of numerous spur shoots, on which alone flowers are produced in the case of the apple and pear. In some of our ornamental shrubs, however, such as Guelder Rose, Weigelia and Forsythia, flowers, are borne on the shoots of the previous year, and it is consequently a mistake to cut these back m the autumn, as is so often done thereby reducing the beauty of these shrubs. Old wood, on the other hand, and branches which have already flowered, should be pruned away at the end of the summer. Some plants which are nearly related, differ in the manner in which they bear their flowers and fruit, and it is therefore very important before pruning to know exactly on which kind of branches the flowers will bic borne. For instance, the Morello Cherry bears its flowers all along the shoots produced during the previous summer, while others flower at the base of the shoots on short spurs. These kinds must therefore be pruned differently from the former. A similar difference is, found in the case of currants. Black Currants flower all along the shoots formed in the previous summer, while Red Currants produce their fruits on small spur shoots found on older wood.
We must distinguish between the methods causing the production of flower buds as described above, and practice such as forcing, which has for its object to cause an early unfolding of the same. Unless bulbs or acrial buds already contain the rudiments of flowers no amount of forcing will cause these to be formed. The application of moist heat, often in faintly lighted pits, is indeed inimical to the production of flower-forming substances and promotes vegetative growth rather than flower production. On the other hand, if flower buds are present their expansion can be materially accelerated. By these means flowers can be obtained in mid-winter when they will be particularly appreciated. Most storage organs, whether tubers, bulbs, or winter buds, require to pass through a resting period before they enter upon a new period of growth. Various methods can be adopted there-fore to accelerate either the process of ripening the buds or of reawakening the dormant structures. While dryness promotes the former, heat and moisture effect 11w latter. has been found however that, the resting period can be shortened by various means. For example particular varieties of potato, which will not sprout in the autumn and cannot therefore be used for the culture, of early potatoes, if placed for a fortnight in a cold chamber slightly above freezing point will readily develop when planted. A similar treatment is advantageous for rhubarb plants which are to be forced. Lift them out of the ground and let them dry and be exposed to light autumn frosts and they will then respond more readily to methods of forcing. This practice of thoroughly cooling plants which are to be forced is now regularly used in the case of bulbs, retarded Lily of the Valley crowns, Lilacs, Spiraeas, etc. Abroad, other methods have come into vogue with the same object in view. It has been found that the immersion of the branches of plants for ten to twelve hours in a hot-water bath of from 85 deg. to 100 deg. Farenheit, has a remarkable effect in accelerating the unfolding of winter buds. The roots should not Ito so treated. It is therefore hest to invert the plants and allow the stem and branches to hang down into the hot water. The effect of the latter is quite local ; so that when partially immersed only those buds which have been under water are affected. Six weeks after this treatment lilacs will be in full bloom, if subsequently grown in the usual way. Professor Johannsen, of Copenhagen, has discovered that exposure of plants to ether vapour for twenty-four to forty-eight hours has a similar accelerating effect. Both methods are largely used on the continent.
Let us now consider the structure of flowers and the function of their various parts. So manifold is the appearance of flowers that it might seem at first difficult in a short space to make any general statement on their structure; yet as regards the essential organs of reproduction we find considerable agreement. It is more particularly in the structure of the brightly coloured petals that we find great variety, and largely owing to the adaptation of the flowers to the visits of insects. The colour and scent is developed to direct them to the honey which the flowers provide, and special honey guides, spots in the Rhododendrons and lines in Pansies and Violas, as well as differences in colour, guide them to the nectaries. In making their way to these the insects come in contact with the delicate stamens of the flower and the pollen contained in the pollen sacs at the tip of the stamen becomes dusted on to the insect. When it visits the next flower, some of this pollen will be dusted off on the stigma at the top of the Immature seed vessel, and thus the insect effects the pollination, which is the necessary preliminary to the fertilisation of the flower. The pollen grains left on the sticky surface of the stigma germinate there, and a delicate tube grows down from them to the immature seeds, which become fertilised by the fusion of the vital element of the pollen grain with that of the ovule. Even though a flower may produce both of these essential organs, the pollen producing stamens and the ovule containing seed vessel, self-pollination does not usually take place as the stamens and seed vessel mature at different times. This can easily be seen in such a large flower as that of the Nasturtium, in which the stamens may be observed ripening one after the other and liberating their pollen, and only after the last of the pollen sacs has shed its pollen does, the three-pronged stigma open and is then ready to receive pollen brought from another flower in which the stamens are opening. All the various and wonderful mechanisms, by means of which flowers ensure. their pollination, have the purpose of securing the fertilisation of the ovules with pollen from another flower and if possible from another plant, for as Darwin has shown cross fertilisation usually causes the production of more numerous and stronger offspring than self-fertilisation.
Gardeners who grow exotic plants, the flowers of which are adapted to the visits of insects not found in this country, have often to perform this service themselves and to pollinate artificially the flowers using usually a fine camel-hair brush. In plants like the Tomato, tapping the stems with a stick, wrapped round, with a cloth to prevent injury to the plant, will generally cause the pollen to drop out of the opened anthers on to the stigma of these pendant flowers. If tomatoes are grown under glass, we must take care that during the period of flowering the house is kept dry, as the pollen sacs will not open in a moist atmosphere and the plants cannot be pollinated. Of course by the method described self-pollination only can be effected, crossing can be obtained by using a fine paint brush and pushing the hairs up between the stamens first of the flower and then of another.
In some plants, such as the Cucumber and Marrow, the flowers are of two kinds, some producing the seed vessel, which can be seen as a thick structure below the petals, and others producing only pollen. In such cases, self-pollination is impossible and cross pollination must be effected either by insects or by the gardener. It is curious that in spite of the many adaptations to cross fertilisation there are many plants in which self-fertilisation is the normal process of seed production. This is the case with most of our cultivated grasses. In the wheat and other cereals, though the flowers open and the light powdery pollen is carried from plant to plant, the stigma of the seed vessel is usually pollinated before the flowers open, and as there is only one seed in each seed vessel it is usually self-fertilised before the foreign pollen arrives. Even in flowers, apparently specially adapted to the visits of insects such as the Sweet' Pea, which possesses both scent and colour, the immature pod is already pollinated in the bud stage of the flower, and if we wish to effect any crossings between different varieties of this plant, we must cut away the stamens before the flowers open and introduce pollen from another plant. While such normal self-pollination has for horticulturists the advantage that it preserves the purity .of the varieties we cultivate, it is probable that it may gradually cause a weakening of the race, and in nature even occasional crossing probably reinvigorates the strain a number of members of the Pea Family are self-fertile, including the commonly cultivated forms of peas and beans, while many other leguminous plants, such as the clovers are self-fertile and cannot be fertilised with their own pollen even when it is placed ,on the stigma of the flower. An interesting analysis has been made of the behaviour in this respect of the various members. of this Family, and it has been found that all the annuals in this group of plants are self-fertile, while the perennial forms are self-sterile. It would seem as if it were of more importance to ensure regular and uninterrupted production of seed in the former, while in perennials, even if in one season owing to the absence of insect visitors, seeds are not produced, the persistence of the plant to the next season when condition's may be more favourable will enable it to reproduce its kind.
The flowers of many varieties of Apple and Pear are self-sterile, and disappointment has often been caused by the failure of a fruit tree to bear when grown singly with no other variety near to it from which pollen could be carried by seed. Even in orchards, if a large number of self-sterile varieties are grown close together, the yield of fruit may not be found to be satisfactory. In both cases it has proved very efficacious to plant near such trees a Siberian or other Crab Apple, which produces a large amount of effective pollen which can be carried by insects from tree to tree.
After fertilisation, when the young plant is beginning to be formed in the seed, the seed vessel too begins to swell, and it is from observing the latter that we can gather that pollination has been successful. Of course the continued growth of the seed-vessel, as well as the development of the embryonic plant and the storage in the seed of enough food material for its subsequent germination, necessitates considerable activity on the part of the vegetative organs particularly of the leaves. It is for this reason advisable in the case of Peas, Scarlet Runners and French Beans, when they begin to set their pods, to encourage their further growth by the use of suitable stimulants such as liquid manure.
It is, obvious that as the formation of seeds and fruits require increased supplies of food material, herbaceous plants that are heavily fruiting will have less food where-with to develop new flower buds, and gradually the plant will cease to produce flowers. If, therefore, we are cultivating plants for the sake of their flowers it cannot be too strongly recommended to pluck off all flowers as soon as they are dead, so that they should not begin to set their seeds. In the case of Sweet Peas, where the seeds take up a large amount of food material, this is particularly important, but it is a good rule to follow in all cases. The fact that a large supply of food material to fruits may exhaust that available for the production of lower buds will explain reduction in the number of flowers on fruit trees, if an abnormally heavy crop of fruit was produced in the previous season.
It is not, however, only the formation of flowers which will be interfered with. Under special circumstances the vegetative organs, too, may suffer.
For this reason it is recommended that young fruit trees, which have been recently transplanted, should not be allowed to ripen many of their fruits during the first season after transplantation, as this may interfere with the proper development of the new root system, which is always damaged to some extent when trees are moved. Similarly, it is advisable when Raspberry canes are newly planted to cut them down to within eight or ten inches of the ground, so that they should not exhaust themselves during the first year, as that would prevent the formation of strong new shoots for the next season.
As ripening fruits require a large amount of nutrition, it is important to see that the stimulating treatment we give to plants which are bearing, should not be used for the production of vegetative shoots. In Tomatoes the growth of such shoots should be checked when the fruits are maturing. It is also advisable to reduce the foliage a little so that the fruits are not shaded. Excessive defoliation, however, will prevent the fruits from attaining their full size as the leaves are the seat of formation for the organic material which the fruits require.
When fully mature the seeds inside the fruits will be found to be surrounded with a hard seed coat, within which we have the minute seedling either containing its store of food material in its fleshy seed leaves as in the Pea or Bean, or surrounded with a supply of nutriment as in the case of the Wheat grain. The food material is usually largely starchy or oily, but in addition there is a smaller amount of the essential nitrogenous material so important to the plant and so essential to man. Of all cereals Wheat is the richest in nitrogenous material, while all leguminous seeds, thanks to the help which their parent plants have received from the nitrogen fixing bacteria in their root tubercles, have a great abundance of organic nitrogen compounds. This is the cause of their great nutritive value to mankind.