General Features Of Plant Life
( Originally Published 1915 )
Nutrition and propagation, vegetative and seed reproduction. Annuals, biennials, and perennials. General account of vegetative organs.
Gardeners being concerned in the cultivation of plants, it is Obviously important that they should be acquainted with their structure and mode of life. They will find it particularly useful to know in what ways plants respond to their environment, for such knowledge will enable them
by varying their treatment to modify the development of the plants they cultivate, to accelerate or retard their growth and to ensure the production of a greater number of flowers. The gardener's aim is not, always the same. The flower lover is anxious to obtain a wealth of bloom. The allotment holder on the other hand, concerned in raising vegetables, may desire well-developed leaves, roots or tubers. It is important therefore that we should study both the vegetative and the reproductive organs.
The former are concerned mainly with nutrition, the latter with the propagation of the plant. These two sets of organs are often differently affected by external conditions. A gardener knows that by over-stimulatmg the development of foliage he may be endangering the production of flowers. It is a common practice to reduce the supply of water so as to encourage the formation of flower buds. Some of our common British fruit trees when grown in tropical climes will develop into luxuriant trees with a wealth of foliage, but lack the flowers essential for the production of fruit. The practice of pruning and root pruning is based on the same phenomenon, and by reducing the amount of vegetative organs the formation of flower buds is encouraged. Yet though' there seems to be this opposition between vegetative and reproductive organs the latter. are really dependent upon the former. . For it is only at the expense of the food material absorbed and worked up by the roots and leaves that the flowers are produced. The activity of the vegetative organs must therefore always precede the formation of flowers.
This is seen very clearly in the case of annuals. These plants complete the whole of their life-cycle in one vegetative season which is usually much shorter than a year, in some arid regions amounting only to a few weeks during and immediately after the wet season. An annual commences as a seedling at the beginning of the favourable vegetative season, and after the production of ,a limited number of leaves produces its flowers, which are kept sup-plied with food material by the activities of roots and leaves until the seeds have matured, then the whole plant dies down to be replaced next season by the offspring developed from its seeds. Such is the life history of the mustard and cress and also that of most of our common weeds like chickweed, groundsel, and some of our grasses. The ubiquity of our weeds is clue not only to their effective means of dispersal, but also to their rapid growth to maturity which enables them to produce two and even three generations of plants in one season.
Slower in their development and exhibiting a more marked contrast of vegetative and reproductive periods are the so-called biennials. In these plants after the vegetative organs are produced they are employed throughout the first summer season in manufacturing and storing a large supply of food material, which is to be used in the formation of flowers and seed during the second year, after which the entire plant dies down having fulfilled its existence and produced a vast number of offspring. For the large store of food material which it has laid up during the first year will enable it to produce a much larger number of flowers than is commonly the case in annual plants. In most biennials the food material is stored in some underground part of the plant such as the swollen ' root ' of the turnip or beet, the leaves during the first season forming a tuft or rosette close to the ground.
Lastly we have perennials which are much more varied in character than the first two groups of plants. These latter are always comparatively soft in texture, but perennials include both herbaceous and woody forms such as trees and shrubs. The herbaceous again are of two types, firstly those which persist throughout the winter like violets and primroses, and secondly those which die down in the autumn leaving a persistent root or root-stock underground, from which the plant renews its growth in spring. Plants of this kind like the iris, peony, larkspur, Michaelmas daisy and many other favourites of our herbaceous borders have like biennials a large store of food material in their underground organs. This enables them in most instances, not only to produce annually a crop of flowers but to branch out underground and develop into ever-spreading clumps, which in many cases require repeated breaking up and thinning just as we require to cut back our bushes and trees. In some cases these underground portions of perennial plants do not remain attached in one mass, but when the plant dies down, a good deal of the underground part dies away too, leaving isolated portions, so that in place of one individual we find many fragments which would seem to be offspring though they are really only remnants of the original parent. Such offsets we have in the case of the tubers of the potato, which represent the rounded swollen ends of underground shoots that have become entirely separated one from the other. Though this is really only a breaking up of the original plant, it is often spoken of as vegetative reproduction but must not be confused with seed reproduction, which is always the result of the fertilisation of flowers. It is important to differentiate these two methods of propagation, particularly as in the case of the potato the tubers used for setting in spring are termed "seed potatoes," though they have really nothing to do with seeds. Vegetative reproduction does not replace seed reproduction but is an additional means of propagation, often of the greatest use both in nature and in cultivation.
The potato, for instance, though it produces flowers in this country very rarely contrives to ripen its seeds in our climate and can only be propagated in England by its tubers, which are indeed the sole reason for its cultivation, for these tubers richly stored with food material are of the greatest importance as a staple food of mankind. The Jerusalem artichoke has the same way of vegetative reproduction and a very similar process obtains in all bulbous plants.
Most perennial plants produce a crop of flowers and fruit every season, in some cases after a shorter or longer period of immaturity as is usual for instance in shrubs and trees. These flowers may make their appearance in spring before the foliage. But in that case the flowers are produced at the expense of the food material built up by the leaves of the preceding summer, while the fruits are generally matured by the activity of the leaves of the same season.
Let us now consider some of the effects of external factors upon the growth of plants. If we germinate a seed under suitable conditions on the surface of the soil we find that when the young root breaks through the seed-coat it bends downward and penetrates into the earth. That this is due to the effect exerted by gravity on the young growing root can be demonstrated by slowly rotating a growing plant on a horizontal axis, when it will be found that the root will grow out horizontally as gravity acts first on one side of the root and then on the other, and thus its effect is eliminated and the root is not affected. The main stem of a plant is equally sensitive to the force of gravity, but responds in a different manner growing in the opposite direction to it and, if laid down horizon-tally, bending upwards at right angles.
Detailed microscopic examination has shown that plants have special regions of perception and it was found by Darwin that as regards its sensitiveness to gravity the seat of perception was the root-tip, and that if this was cut off the root ceased to respond to gravity. In the stem it is known that the perceptive region is not so limited in extent. But while the main root and the main stem tend by their response to gravity to grow in a vertical direction, lateral roots and lateral branches do not respond in a similar manner but tend to place themselves more or less horizontally or obliquely. A most curious feature of plant life, and one very difficult of explanation, is the fact that when the tap root or the, main stem of a plant is destroyed, a lateral branch will take its place and assume the vertical position. The importance of this position is self evident when we consider the functions of the root and of the stem. The former acts as the absorbing organ, extracting from the soil the water and valuable salts necessary for the growth of the plant; it is obviously essential therefore that the root should grow downwards in search of moisture. It is also important that the lateral roots should not grow in the same direction as the main root, so that they can search out other regions of the soil in their quest for food material. The spreading habit has the further advantage that it anchors tie plant more firmly in the soil and pre-vents it from being easily uprooted by the wind.
The stem on the other hand growing away from the soil is in an advantageous position for exposing the leaves it bears to the full rays of the sun and thus enabling them to fulfil their main function in the life of the plant. This function is to absorb as much light, as possible and by converting the light rays into energy to build up the organic material upon which the formation of flowers and fruits depends. Light being therefore of such vital importance to plants we find that in the course of their evolution they have acquired the power of responding in their growth to the stimulus of light. It is a familiar fact that stems of plants will bend towards the light if instead of being illuminated from all sides they receive the light from one side only. Growing in the open the main stem is erect and the leaves are usually expanded horizontally, that is at right angles to the direction of maximum illumination which is from above. When shaded on one side the stem inclines towards the light and the leaves are placed obliquely, often indeed horizontally, when a plant is grown in a window, in which case it is more or less shaded on three sides. Roots as they are underground do not normally exhibit in their growth any response to light, but if a young seedling plant is grown with its roots in a glass jar it will be found that if illuminated from one side only the roots will bend away from the light. In nature this can be observed in the roots which grow out from the stem of the ivy when clinging to a wall. They are generally produced from the shaded side of the stem and always bend away from the light and towards the wall. These climbing roots are not sensitive to gravity as they have a very definite function to perform in fixing the plant to the wall, and are not concerned in the absorption of food material.
Wherever bending, such as described above, takes place in growing organs, this is due to differences in the amount of growth on different sides of the stem or root. The concave side grows less than the convex side. Thus when a stem, which has been laid horizontally, bends upwards, this is due to the greater amount of growth of the side nearer the ground. If a stem illuminated from one side bends permanently in that direction this is due to the fact that light retards the rate of growth and the side away from the light growing more rapidly the stem becomes convex on this side and bends towards the light. The fact that light. retards growth and therefore causes plants to be short and " stocky " is of course a well-known phenomenon, while the lack of illumination acting like darkness causes more rapid growth and we get long " leggy' plants, when they are insufficiently lighted as when grown in the shade, in deep frames or pits, or not close up to the lights in greenhouses.
There are one or two other factors influencing the growth of plants which it may be useful to refer to at this juncture. Besides being sensitive to gravity roots are also sensitive to contact, and when a root tip comes in with a solid body such as a stone in the soil it bends away from it. This is brought about as in the case of other bending movements, by the fact that contact causes retardation of growth on this side of the root and this side becomes therefore convex, the root-tip pointing away from the obstacle met with in the soil. In this way it is possible for the root to make its way even through a stony soil, avoiding or rather growing round all obstacles with which it may be met, bending to right or to left in its progress downwards. This sensitiveness to contact is however not only possessed by main roots which grow downwards ; it is equally important to lateral roots.
It has been found in the somewhat infrequent cases occurring in nature that where the soil is drier on one side of a plant than on the other the root system develops more abundantly in the moister soil. Experimentally, too we can prove that a root will grow towards moisture just as the stem of a plant will grow towards the light.
Lastly, we have plenty of evidence that in addition to their need for water the roots of plants, just like the living parts of all plants or animals require for their growth that life-giving constituent of the air, which we also breathe in, namely, oxygen. This need of the roots for air is one of the fundamental facts which governs not only the distribution of plants in nature, but dominates our whole agricultural and horticultural practice. It is this need for air which causes the farmer to drain his fields and the gardener to grow his plants in porous and well-drained pots. If we turn a plant ont of such a pot we shall see by the dense felting of the roots in close contact with the sides of the porous pot and among the crocks at the bottom of the pot how eager the roots are for air. If on the other hand we do not secure drainage or if we over-water pot-plants we soon find that they show signs of ill-health. As a matter of fact their roots are being suffocated and may die away unless we alter our treatment. Probably more plants are lost by over-watering than by insufficiency of water. Normally, therefore, though water is a prime necessity of plants its provision must not interfere with the respiratory process o f plants, and we must always provide a porous soil for our crops, that is a soil with sufficient air. This is one of the reasons for the practice of hoeing. For apart from the clearing out of weeds, hoeing prevents the ground from becoming caked, a condition which would prevent the free access of air into the soil. At certain stages in their growth this need seems greater than at others. Speaking generally we may say the more actively growth is proceeding, the greater the need for air. Germinating seeds for instance require a large amount of air, and when the formation of new roots is proceeding in the case of cuttings and layerings a porous soil is essential. When a potato starts its new growth and. is rapidly developing its new shoots and roots we. find that the skin, a hard and impervious layer, becomes interrupted by numerous breathing pores which enable an active respiratory process to take place. These pores can easily be seen with the naked eye on the skin of a sprouting potato. They are equally clearly seen on the twigs of trees such as those of the horse-chestnut. On the leaves the pores through which the plant takes in various constituents from the air are not visible except with such magnification as the microscope affords.
As the leaves of most plants arc expanded in a drier medium than that which surrounds the roots, these pores naturally allow a large amount of' moisture to escape particularly in dry weather. If we cover a plant with a bell-jar we can see this moisture condensing on the sides of the glass, and in carefully conducted experiments we can actually measure the amount of water so lost. From such calculations we estimate that a fair-sized tree standing in the open may lose ten gallons of water in the course of a summer's day. This loss of water from the leaves might at first be thought to be an accidental phenomenon due to the possession by plants of delicate expanded leaf structures. As a matter of fact, however, this process of transpiration is of vital importance to the plant. In the first place it can be shown by experiment that leaf transpiration exerts a considerable amount of suction, and it is in part by this means that water is raised in tall trees to the upper branches. In shrubs and herbaceous plants this suction alone would be sufficient to raise the water into the leaves and flowers, though the roots too are equally concerned in the ascent of sap. Nor must we consider the water which is given off by the leaves as so much waste. It is indeed important that some of the water absorbed by the roots should be driven off. For the water contained in the soil contains the mineral salts which are necessary for the plant, in very dilute solution, and these salts require there-fore to become concentrated in the plant. This can only be done by driving off some of the superfluous water and that is effected by the process of transpiration. The leaves are therefore acting as a condensing apparatus and thus performing a very needful function.
I trust that what I have said regarding the sensitiveness of the various parts of the plant to external forces and surrounding conditions will have impressed the reader with the power of response inherent in plants, and this will indicate how by artificially selecting our medium and method of treatment we can considerably modify the course of development of the plants we cultivate.