Stems And Leaves
( Originally Published 1915 )
Mechanical requirements of stems and branches. Twining and climbing plants. The food conducting function of stems The work of leaves and their structure. The wilting and recovery of leaves. Protection of leaves against excessive drought. Direct effect of surroundings on leaf development. Hardening off plants.
The stem Is not as Important an organ of plants as are the roots and leaves. The latter are essential for the nutrition of plants, and their particular work can only be carried on if the leaves are fully exposed to the sunlight.
There is consequently considerable competition among plants for " a place in the sun," and in the course of evolution there has been a development of plants of increasing size, which by overlapping the smaller ones have been able to reach the light. This has led gradually to the production of trees and shrubs, but in herbaceous plants too, the stern and branches have the function of displaying the leaves to the best advantage. This and certain mechanical principles which are involved account for the peculiarities of branching typical of different plants. In addition to the stem and branches the leaf stalks are also concerned in the ultimate positions in which the leaves are expanded. By varying growth in length and direction the leaves become so placed that there is little or no overlapping and the dovetailing of the various leaves causes the formation of a regular " leaf mosaic."
This is very clearly seen in the arrangement of the leaves on the horizontal branches of a tree, or in the case of ivy growing up a tree trunk or on a wall. Even in plants possessing no stern, and in which therefore the leaves are found close to the ground forming a rosette, the leaves are found to overlap very little and particularly when they are stalked it can be clearly seen that young leaves which are formed near the middle of the rosette do not overlap the older leaves, as the latter become pushed out more and more from the centre by the elongation of the leaf-stalks.
The function of the stem being to bear and display the leaves it follows that, to carry the foliage and to resist the strain which winds will exert upon leafy plants the stem has to meet certain mechanical requirements. The rigidity, and at the same time the elasticity of the stein is attained by the development of certain thick-walled cells which collected into groups are so arranged as to give the greatest amount of strength with the smallest expenditure of material. These strengthening cells arc displayed in plants on the principles which have been adopted by engineers in the manufacture of rigid, and at the same time elastic structures. We have, firstly, the hollow cylinder as is seen in grass haulms. The slender grasses can maintain their erect position by virtue of the mechanical properties of their straw. The bamboo cane is perhaps the most powerful of all in this form of construction. In other cases we find the stems developing internally a system of girders which give them the strength they require.
There are some plants in which the stem has not sufficient rigidity to grow erect as it does in most instances.
In such cases the stern may trail along the ground, often covering a large area and the plant is then often modified as a shade plant unless it grows among very short vegetion, as in the case of some mountain plants. Sometimes by using the support of the rigid stems of other plants twiners and climbers are able to reach the light though they have to compete with much stouter vegetation.
Twiners, such as the scarlet runner and the hop, have no special climbing structures, but their slender stems ending in a heavy terminal bud are bent over to one side, and on close examination can be seen to rotate very slowly, either clockwise or counter clockwise. This circular movement of the top of the stein causes it to grow round any upright or nearly vertical object in its neighbourhood, and then as the slender stern tries to straighten itself out it grips firmly the support round which it has grown. As the rotation is always in the same direction for a given species, it is necessary before giving such a plant any artificial aid to see in which way the rotation takes place, for if one twines the plant in the wrong direction it will unwind itself again when left alone. It is difficult to keep a twining plant to a horizontal course as it of itself will never twine round a horizontal support. Climbing plants on the other hand can fix their special climbing, organs or tendrils to horizontal supports, partly owing to the sensitiveness of these organs to contact. In some cases, as for instance in the Virginia Creeper (Ampelopsis) the contact stimulus causes the tips of the tendrils to swell up into sticky suckers, which enable this creeper to fix itself to a vertical wall. More frequently however when the tendrils come into touch with some object, they grow round it by virtue of the fact that the side which is touched grows less rapidly, so that it curves round the support. After it has grasped the latter the tendril contracts spirally, and thus tightens the climber to its support. Tendrils are generally formed from parts of the leaf, the leaf-tips in peas, the leaf-stalks in the Nasturtium and Clematis.
Besides carrying the leaves the stem has the further function of conducting to these leaves the food material absorbed by the roots. As we have seen the forces at work in connection with the supply of water to the leaves are, firstly, the root pressure (lecture 2), and secondly, the transpiration current in leaves (lecture I), which latter causes a considerable suction to be exerted on the water in the stem. The special channels through which the sap rises are the vessels, long continuous passages, running in the wood of the stem. But this upward passage of water with the inorganic salts it contains, is not the only conduction required. The complex substances formed, as we shall sec, in the leaves require to he conducted away partly to nourish the flowers and fruits, partly to enable further growth and development of the roots to take place. This elaborated sap passes through other channels which lie outside the wood and are spoken of as the bast-tubes. These are very delicate, and to prevent them from being crushed they are protected on the outside by strong resistant fibres called the bast fibres or hard bast.
With regard to the essential work of leaves their primary function is without doubt to build up organic food material from the water taken up by the roots. This they are able to combine with carbon which they take from the atmosphere, where it occurs plentifully in the form of carbonic acid, the gas we breathe out from our lungs. In the presence of sunlight all parts of the plant which contain the characteristic green colouring matter are able to break up the carbonic acid, retaining the carbon and giving back the .oxygen which was previously combined with the carbon. In doing this they are constantly purifying as it were the air and enriching it with the life-sustaining oxygen needed by man and all forms of animal life. The carbon which the plants retain is combined with the elements of water to form organic material called carbohydrates, of which starch and sugar are the most important to plants.
Within certain limits the brighter the light the more active is this process of carbon nutrition, or assimilation as it is called. Hence the great importance of giving plants as bright and sunny a position as possible, and hence also the reason why plants seek to secure the most favourable positions for their leaves. The formation of leaf mosaics, mentioned above, is a good instance of means adopted to secure the most effective display of leaves. Unless specially adapted for growing in shade, as are for instance most ferns, the majority of plants will not show their best development unless grown in an open situation. Apart from the essential influence of light upon the process of leaf nutrition, it has been found by experiment that this important nutritive process is affected by temperature being within certain limits proportional to the rise in temperature. This explains why in sheltered places, where the air is heated up in winter many plants are known to make better growth than actually in the open. Shade can be partially counteracted by warmth. In frames in which plants often get less light than in the open, the warmer temperature enables them to grow more rapidly than were they exposed to the colder air. It must not be forgotten too, that not only is the process of leaf nutrition stimulated by warmth but root-absorption also increases with a rise in temperature, so that all nutritive processes are favoured by warmer conditions of soil or climate.
A secondary but important function of leaves was referred to in a preceding lecture, namely, the importance of getting rid of a certain amount of water vapour into the atmosphere. This process of transpiration aids the upward passage of sap and also causes the concentration of the necessary mineral salts, which are only contained in dilute solution in the soil.
Let us now consider how the structure of leaves is adapted to perform its important functions. For both of them it is essential that the air should be in close contact with the cells of the leaf. It is essential to prevent too great a loss of water from the leaves and also for protective purposes it is necessary that the surface of the leaf should be covered with a more or less resistant layer. Consequently the passage of air to the interior of the leaf is effected through microscopic pores called stomata, which are usually found in greatest number on the under surface of leaves. These pores have the power of opening and closing so that the aperture can be regulated to the needs of the plant. They open with the approach of daylight and thus enable the carbonic acid to enter the leaves when the conditions arc favourable for leaf nutrition. At the same time, of course, water vapour escapes outwards through the pores. The inner parts of the leaf are so arranged that the cells containing most of the green colour-mg matter are near the upper surface of the leaf where they will receive most light . Throughout the interior of the leaf there are wide spaces through which the air can circulate readily between the cells. The veins which bring up the supply of water and which also have to collect and conduct away through their bast portion the organic material formed by the leaf, become very finely branched so as to be in contact with all parts, of the leaf.
If we consider that the larger the leaf surface the larger the amount of light. absorbed and hence the greater the nutrition, we might wonder why all plants have not larger leaves. The limitation in the size of leaves is largely due to mechanical considerations. A slender stem cannot bear leaves beyond a certain size. Moreover, though a large leaf would increase the process of carbon assimilation, the loss of water from a large leaf surface might be more rapid than the absorption of water by the roots. Whenever this happens it causes the wilting of drooping of leaves, and should that occur too frequently the plant will not survive. As it is, wilting of leaves the open occurs in hot summer days and affects main plants with large delicate leaves, large at all events it proportion to their roots, such as Doronicums, Calceolarias, and young Cauliflowers. In the case of pot plants, which are not only losing water through their leaves but also through the sides of the porous pots, it is a common phenomenon to find plants wilting during the heat of the day, and to prevent this it is necessary to shade the house during the summer months, at least on very bright days Wilting is particularly noticeable in recently transplante plants, in which there is always sure to have been sonic injury to the roots so that absorption cannot keep pace with transpiration. Transplanting is therefore best undertaken in wet or dull weather, or at the end of the day, whet during the succeeding night the stomata of the leaves will be closed, and consequently at the start, at all events, there will be no undue loss of water. Cuttings which have not yet produced an adequate supply of roots require it) many cases to be kept in a moisture-laden atmosphere in a frame. It is interesting to note that when plants in tilt open flag at noon on a hot summer's day, they recover again at night even without further water supply. Where the leaves droop the stomata usually tend to close, and at night they are certainly closed so that the roots have then the chance of replenishing the drooping leaves wit l i water. Watering the plants while the sun is still on them should be avoided. Drops of water on the leaves are apt to act as lenses and focussing the rays of light upon. tilt leaf often cause burning of the tissues and consequent spotting of the leaves. Watering the soil has also some dangers unless properly done. Merely damping the surface of the soil, apart from the tendency to cause it to cake, stimulates the growth of surface roots, which will constantly require watering in dry weather. It is better to give plants a good soaking from time to time. The water penetrating to some depth causes the development of deep roots, which will keep the plant supplied with water even in dry weather.
It is often thought that leaves, or at any rate the leave of some plants, have the power of absorbing water. It is not an uncommon practice to spray the leaves of plants in the greenhouse when they are shaded, and also in the open when the sun is no longer upon them and their subsequent recovery, if they were drooping before, is attributed to their having absorbed water. This is however not the case. The spraying of the leaves, besides cooling the foliage, which is often considerably heated by the sun, has the effect by the evaporation of the water to render the atmosphere around the plant moisture-laden, wherefore the transpiration of the leaves is at once decreased.
It is this decrease of transpiration and not absorption by the leaves which causes the revival of the plants. No leaves with an impervious covering can take in water and there are very few leaves which are not so protected.
Mosses and Filmy Ferns alone among leafy plants can take in water through their leaves, which are adapted to growth in moist if not dripping conditions.
It is interesting to note that when the stomata are closed and transpiration is impossible, some plants are still able to get rid of their superfluous water. At night the leaves of the Nasturtium and the Fuchsia for instance are able to force out little drops of water through waterpores, which are found at the termination of their veins. Many grasses, too, have water-pores at the tips of their leaves, and the drops of water exuded by them often look like the formation of dew.
When growing in very dry soil or in hot arid climes, plants may experience a great difficulty in supplying their leaves with water, often during a very prolonged period.
In such cases many remarkable deviations from the normal structure are produced, with the object of protecting the leaves against draught. In most cases the leaves are small, often reduced to mere scales or needle-shaped structures.
Such are the leaves of many of our moorland plants growing in sandy, well-drained soil and exposed to strong and drying winds. In still more arid regions the leaves may disappear altogether, and we get such curious plants produced as the fleshy Cacti of the New World and the succulent Euphorbias (Spurges) of the Old World. In both of these groups of plants the green stems undertake the process of assimilation, and they also store up sufficient water during the wet season to enable the plant to last out during a prolonged drought.
It is sometimes stated that these strange modifications are the result of the direct action of the environment on vegetation. This is impossible of proof, and we may assume that it is largely by natural selection of forms or varieties most suited to these extremes of climate that desert plants have in the course of ages sprung from ordinary types of vegetation. Nevertheless we must not forget that to some extent climate has a direct modifying action. If, for example, we grow a seedling gorse in a moist greenhouse, we find that it will persist for a long time in producing small ordinary leaves, which precede the spines in the young plant, and this effect is certainly a direct action of the surrounding conditions. It is also well known that the texture of leaves produced by plants grown in a greenhouse differs greatly from that of leaves of the same species growing out of doors. It is necessary therefore before transplanting into the open plants raised under glass, to get them gradually acclimatised to their new surroundings. By transferring them from greenhouse to frame and keeping the latter well ventilated, or by placing plants in a sheltered place in the open, the leaves have an opportunity of becoming hardened to more rigorous conditions of existence. The outer layer of the young leaves can become strengthened, and they as well as the new leaves will all be. modified to suit the new environment. In transferring a plant from a dry to a very moist house it will often be noted that the older leaves are shed. Being fully developed they are unable to adapt themselves to the new conditions. Not able to transpire freely owing to the moisture-laden atmosphere they become overcharged with water and this probably causes them to fall. The new leaves will be suitably modified to suit the more humid conditions.