Читать книгу Field and Woodland Plants - William S. Furneaux - Страница 17

Table of Contents

The work of the botanist is light during the early spring, especially if his attention is directed only to plants and trees in their flowering stages; but, to one whose ambition is to study Nature in all her varied phases, this season of the bursting of the bud, when all things are awakening into new life, is full of interest, and demands no small amount of time.

The first flowers observed in the spring are mainly those hardy weeds which may be seen in bloom almost through the year, such as the Shepherd's Purse, Chickweed, Groundsel, White Dead Nettle, Red Dead Nettle, and Henbit Dead Nettle. These are soon followed by the Furze, Strawberry-leaved Cinquefoil, Snowdrop, Hazel, Common Whitlow-grass, and other flowers that are truly blossoms of the spring. All these will be described in turn, according to their various habitats; the object of the present short chapter being to note those signs of early spring which demand the attention of the lover of Nature while flowers are as yet few and inconspicuous.

A ramble over bleak downs and moors during the cold days of early spring will probably reveal but little of interest in the way of vegetable life, but in sheltered vales and woods, copses, and protected waysides, there is much to be observed. Here it is that we find the hardy weeds which have continued to bloom throughout the winter months; the earliest of the spring flowers; the fresh green foliage of herbs and shrubs that, in more exposed situations, have been completely denuded; the first tender seedlings appearing above the ground long before the frosts are over; and the expanding 'leaf-buds' showing their green while elsewhere all life seems dormant.

This is the season when the young botanist requires his notebook more than the collecting-book or vasculum; for his records of early flowers, and of the times of the appearance of the leaf in our trees and shrubs, will prove of great interest when compared with the corresponding events and times of other years. Not only do our spring seasons vary considerably from year to year in such a manner as to alter the general times of appearance of leaf and flower, but the vicissitudes of our climate even change the order in which these events occur.

The general study of the buds of trees should commence before they begin to burst. We commonly speak of the buds as winter buds, but it should be known that they were formed in the preceding summer or autumn, and have remained dormant throughout the winter. There is usually a terminal bud at the tip of each twig, and lateral buds at the sides. If we examine a lateral bud we find immediately beneath it a more or less distinct scar, denoting the position of a leaf that fell in the autumn, thus showing that the bud in question was formed in the axil or angle of the leaf. These observations should be verified by examining the trees in autumn, while the leaves still exist.

It is not sufficient that we are able to recognise trees when in leaf; they should be known equally or almost as well during the winter and early spring while the branches are bare, and this is usually easily accomplished by making ourselves acquainted with the general form of each tree as viewed from a distance, and, on closer inspection, with the nature of the bark and the character of the buds.

All our forest trees are of the exogenous type; that is, their stems increase in thickness by the addition of new wood formed outside the older wood and underneath the bark. Thus the bark, which is composed of a layer or mass of dead, sapless cells, is gradually pushed outward as the stem thickens. The result is that the bark is either more or less fractured, as in the Elm and the Oak, or it flakes off and falls to the ground, as is the case with the Plane and the Birch. A new layer of bark is always formed during each summer, and this, in turn, either cracks or peels away; but while, in the former instance, the accumulated bark presents a very rugged appearance, and becomes very thick, in the latter case it remains smooth, and is always thin.

Then again, how are we to account for the great variety in the general forms of our different trees—the irregular, crooked nature of the Oak; the slender, but denser branching of the airy Birch; and the tall, pyramidal form of the Lombardy Poplar? All this is easily understood if we carefully observe the positions of the buds as seen during the winter months; and watch the development of these buds during early spring.

Trees in Winter or Early Spring

1. Hazel, with catkins. 2. Ash. 3. Oak. 4. Lime, with remains of the last season's fruits.

If the buds are irregularly scattered on the twigs, the lateral buds being as strongly developed as the terminal ones, while, in the spring, as is often the case, certain only of the buds develop into new twigs, the others remaining dormant, then the branches assume that irregular, crooked appearance so characteristic of the Oak. If, on the other hand, all the terminal buds are well developed, and the lateral buds are weaker and more regularly distributed, but farther apart, then the tree grows more rapidly in height than in breadth, and assumes more nearly the character of the Pyramidal Poplar. It will thus be seen that the study of trees in their winter condition is not altogether lacking in interest.

Referring once more, but briefly, to the matter of dormant buds, we recommend the reader not only to observe that some buds do not expand with the others during the spring, but to make them the subject of experiment. Thus, when the Horsechestnut is well in leaf, dormant buds will usually be seen on the sides of the twigs, sheltered by the spreading leaves produced at the tips. Now remove the whole cluster of leaves formed by the terminal bud, together with the bud itself, and the hitherto dormant laterals, under the influence of increased light and warmth, and supplied with sap that is now directed into new channels, will speedily show signs of growth. Similarly, the fruit-gardener will remove the tips of the branches of his fruit trees, which often bear buds that are destined to produce leafy twigs only, and thus encourage the growth of the fruiting buds that are situated lower on the twigs.

Let us now briefly consider the structure of buds and the manner in which they are protected. Most buds are surrounded by brownish scales which are impervious to water, and thus prevent a loss by evaporation at a season when the activity of the roots in absorbing moisture from the soil is suspended. Such loss is still further insured in some cases by a covering of natural varnish. On removing this protective coat we find a dense cluster of closely-packed leaves, variously folded or crumpled in different species, and often, in the centre, a cluster of flowers.

What, then, is the true definition of a bud? It is a young branch, and may give rise to a mature branch bearing foliage leaves only, floral leaves only, or a combination of both. A transverse section of a bud, examined, if necessary, with the aid of the microscope, will show the nature of the branch it was destined to produce; and, in the case of buds which represent, in embryo, branches bearing flowers, or both leaves and flowers, it is often an easy matter to see the whorls of the future flowers, and even the pollen cells in the anthers and the ovules in the ovary.

Trees in Winter or Early Spring

5. Birch, with catkins. 6. Poplar. 7. Beech. 8. Alder, with catkins and the old fruit 'cones' of the previous season.

Interesting as it is to study the structure of buds in their dormant condition during winter and early spring, even more fascinating is the watching of the gradual expansion of the bud and the unfolding of the young leaves. And it is not always necessary to make frequent visits to the woods in order to carry out such observations, for a large number of buds will develop almost equally well, at any rate through their earlier stages, if the twigs bearing them be placed in vessels of water either in or out of doors; and in many cases all the stages from dormant bud to perfect leaves and fully-expanded flowers may be watched in this way.

We have spoken of the protection afforded to the dormant bud during the winter period, but it is interesting to note that protection is necessary for the young leaves even after they have forced themselves well out into the light and air. The reason for this is that the epidermis or outer skin of the young leaf is not properly developed. It is not yet water-tight, and, consequently, the sap of the tender leaves would rapidly evaporate, so that they would soon become dry and shrivelled.

The means by which the young leaves are protected will be readily seen if we watch the gradual development of the bud. In many cases these leaves remain folded long after they have left the shelter of the original bud-scales, the manner of folding being the same as that which obtained while within the bud. Sometimes they are folded like a fan, or like the leaves of a book; sometimes rolled one within the other, or irregularly crumpled in such a manner that nothing is exposed to the air except the edges of the leaves and the surfaces of the veins.

In addition to the protection from evaporation afforded by the folding of the young leaves, many are covered with a dense coat of "wool." Young leaves of the Horsechestnut are very thickly covered with such a coat, of which only the slightest traces are to be seen in the fully-grown leaf. The young leaves of the Beech are folded like a fan for some time after they have left the enclosure of the bud, and the folding is such that the only parts exposed are the margins, the midrib, and the strongly-marked parallel veins. But since all these parts are provided with hairs, the young leaf, as long as it is folded, is surrounded by a complete protective covering. As the epidermis develops, and the danger of loss by evaporation thus reduced, the leaf straightens itself out, and the hairs either fall or become shrivelled. The leaf of the Wayfaring Tree is protected, while young, by a complete covering of starlike hairs which form a fine felted coat over the whole surface; and when the epidermis is properly formed, the hairs are all shed.

Some young leaves are preserved by scaly stipules which surround them after they have emerged from the bud; and as soon as the epidermis is sufficiently impermeable the stipules, having done their work, fall to the ground. So great is the shower of these transient structures, in the case of the Oak, Elm, and Lime trees, that the ground is almost completely covered by them.

Twig of the Lime in Spring, Showing the Deciduous, Scaly Stipules.

Young leaves have yet another way of preventing the evaporation of their sap, and that is by turning themselves into the erect position so that the warmth of the spring sun has but little effect on them. The young leaves of various grasses turn their apices upwards; while those of the Horsechestnut, after having lost the protection afforded by the woolly covering and the original folding, turn themselves with their points downwards. Later, when the epidermis is well formed, and the leaves are so far developed that they are capable of utilising the energy of the sun in the performance of their functions, they take up the horizontal position.

Another interesting matter for spring observation is the relative times of the bursting of the flowering buds and the leafing buds on the same species of tree or shrub. In many cases the former are fully developed before the latter show any signs of active growth, or while the foliage is as yet only passing through its earliest stages. The Hazel catkins shed their abundance of pollen before the foliage buds show the slightest signs of green. The Blackthorn is white with snowy blossoms before a leaf appears. The upper twigs of the Elm appear fluffy in the distance through the formation of its flowers while the foliage buds are still dormant; and the Alder, Willow, Poplar and Aspen likewise produce full-blown catkins while their branches are otherwise bare. Of the trees above named, the Hazel, Elm, Alder, Poplar, and Aspen are dependent on the spring winds for the transfer of the pollen, but the pollination of the Willow and the Blackthorn is brought about by the agency of early insects which visit the flowers for the nectar they provide.

Seedling of the Beech, Showing the Cotyledons and the First Foliage Leaves.

The same spring sun which calls forth the new leaves and early flowers exerts its vivifying influence on the seeds that fell to the ground before the winter's frosts set in, and in sheltered places myriads of young seedlings of plants and trees may be found in their first stages of growth. The early history of a plant is as interesting a study as that of the mature specimen, and the young botanist will do well if he seeks out the germinating seeds and watches their development. This part of botanical study may, perhaps, be carried on more conveniently at home than in the field, for the seedlings may be grown in soil, wet sawdust, or in water alone, and the stages closely observed.

The seed is a plant in embryo. It consists of a young root, a bud, and one or two seed-leaves or cotyledons. Some seeds contain nothing but the parts just named, and when this is the case the cotyledons contain a reserve of food material sufficient to maintain the developing plant until the root is enabled to absorb sufficient nutriment from the soil, and the first foliage leaves are so far advanced that they can absorb carbonic acid gas from the air, and build up with the aid of this gas, together with the food obtained from the soil, the compounds required by the growing plant.

Other seeds contain, in addition to the embryo, a reserve of nutrient material quite distinct from it; and in such instances the cotyledons have the power of taking up this reserve, changing it to a condition suitable to the requirements of the plant, and then distributing it to the growing parts.

In some seedlings the cotyledons will remain for some time within, or partially within the seed, in order that they may continue the absorption of this reserve; and while this process is going on the seed may remain below the surface of the soil, or it may be lifted into the air by the upward growth of the cotyledons themselves.

In cases where the cotyledons contain the food reserve for the seedling they sometimes remain under the soil, but in many instances they are pushed into the air by the upward growth of that portion of the plant axis immediately below them. In either case they decay as soon as their work is accomplished. This often happens as soon as they have delivered up to the seedling their reserve of food, but frequently the cotyledons which ascend into the air expand, becoming really leaflike in general appearance, assuming a green colour through the development of chlorophyll (the green colouring matter of plants), and then perform all the functions of the ordinary foliage leaves of the plant. Such cotyledons often continue to exist long after the first foliage leaves have appeared from the bud, for, although the original food reserve has been exhausted, they are now in a position to manufacture, under the combined influence of the sun's warmth and light, compounds essential for their own growth as well as that of the other parts of the seedling. These cotyledons, however, are never of the same form as the true foliage leaves.

The student should obtain a variety of seeds or seedlings of our wild plants and forest trees in order to study these interesting early stages. Such employment will prove very valuable at a season when there is but little call for outdoor work.