Aspects of plant life; with special reference to the British flora

by R. Lloyd Praeger

The STEMS of plants may be looked on from two points of view—as a framework devoted to the display of the leaves and flowers, and as pipe-lines connected with the nutrition of the plant, conveying raw materials from the roots to the leaves, and manufactured products from the leaves to all growing parts. It is the former relation which has mainly determined the forms of stems. Even a very slender stem can convey a vast amount of water and food to a plant which is transpiring or growing actively, as we can test roughly by weighing a pot shrub as it begins to come into leaf, and again a week later, or comparing the growth of a pea with the size of its stem at the base. The surprising variation in length, thickness, form, position, and branching of stems is the plant’s response to external conditions—such as exposure, the competition of neighbouring plants, and so on—which resolve themselves ultimately into questions of wind-pressure, of temperature, of moisture, and in particular of light. The first duty of most stems is to spread out the leaves so that they may receive a maximum share of sunlight, and the complicated systems of branches with which we are so well acquainted are devoted to this object, the leaves themselves helping materially by the positions which they assume. This familiar and typical kind of stem, upright and column-like, beautifully constructed to bear the weight of leaves and branches, and to resist wind-pressure, alone furnishes a delightful study; but it can be dealt with only very briefly, as also some of the modifications which it undergoes under special circumstances.

To plants which have not taken to a terrestrial existence, and which still inhabit their ancestral home in the water, the stem problem is comparatively simple. A flexible shaft capable of withstanding wave and current action suffices so far as mechanical considerations go; such shafts—as we may observe by watching the Oar-weed (Laminaria) on an exposed coast—are effective under very arduous conditions. Those Seed Plants which, evolved on land, have later returned to the water, such as the Pondweeds (Potamogeton), have often redeveloped a stem of a similar kind—a flexible shaft possessing a sufficient tensile strength. The specific gravity of such plants does not exceed that of the medium in which they are immersed, and the stem has not to support the weight of leaves and branches. It is, therefore, not surprising to find that the longest, though by no means the bulkiest, of all plants, are found in the sea. Some of the Oar-weeds (Macrocystis) of the southern and western oceans attain lengths which have been estimated at 500 to 1,000 feet; but these gigantic Seaweeds are nevertheless slender plants, suspended lightly in the water. But after the colonization of the land by the aquatic flora numerous serious problems had to be encountered and solved before plants in an aerial environment could rise boldly into the air. Extremes of temperature unknown in the water had to be faced. Along with a greatly increased loss of water owing to the presence of air and direct sunlight, the area over which water might be absorbed became largely reduced, the roots alone being now available. The whole weight of branches and leaves and fruit had to be borne by the stem, not only in calm but in storm. No wonder that to meet these conditions, or to avoid such extremes as were avoidable, aerial stems often display great complexity and diversity of structure and form. From the mechanical standpoint the tall stem is especially interesting on account of the

Fig. 19.—Arrangement of Strengthening Material in Root (a) and in Stem (b) (DIAGRAMMATIC).

beautiful structural adaptations by which it meets the various stresses to which it is subjected. The problem before the plant is to combine a minimum quantity of material with a maximum of strength and rigidity. Strands of toughened fibre, so disposed as to meet the stresses most advantageously, are characteristic of such stems. In the case of many tall annuals, such as the larger Umbelliferæ, the principle of the hollow column is largely employed; in proportion to the strength obtained, this is far more economical than a solid column: and economy is particularly necessary in such annual stems, where the time available for construction is short. Transverse partitions at intervals provide stiffening of the whole; and as the efficiency of the toughened longitudinal strands increases with their distance from the centre, the stems are often ribbed, the strands occupying the ribs, with softer substance between. This form of construction may be contrasted with that obtaining in the roots. In the latter the greatest mechanical stress is in the form of a longitudinal pull caused by swaying of the stem under wind-pressure. To meet this the vascular strands are arranged, not marginally, but in a central bundle, where they can best meet stresses of the kind. In most trees the stems are solid; here economy of material is less urgent, as a long period of years is available for their building up; the great amount of cell-space thus made available for food-storage is a valuable asset to the plant, as is evident from a consideration of the vast amount of fresh tissue produced in a brief period by a deciduous tree when it bursts into leaf. As this material, stored in the stems and roots, has to be sent up to the twigs dissolved in water, and as during the whole period of growth vast amounts of water are transpired, an elaborate and complete pipe-system is intercalated with the reinforced-concrete structure of the tree trunk. Pumped up by the roots, and sucked up by the leaves, water and food pass rapidly from the ground to the topmost twig of the loftiest tree.

To explain the massiveness of a tree trunk we have to remember that, while the cross-section of any structure varies as the square of its linear dimension, the volume varies as the cube of the same. If we double the dimensions of a tree, we increase its weight eight times, but the strength of the trunk is increased only four times. If a tree 100 feet high is supported on a stem 6 feet in diameter, a tree 200 feet high of the same proportions would need a stem not 12 feet, but over 17 feet in diameter, to be supported equally efficiently. This proportion increases rapidly: a similar tree 300 feet high would need a stem 30 feet in diameter; a tree 1,000 feet high would require a stem 180 feet in diameter, or 32,400 square feet in cross-section. We see, then, why a limit of tree growth is rapidly reached, at about 300 feet, and why the trees which grow to that height have trunks which are one of the wonders of the world, exceeding 30 feet in diameter, or about 100 feet in circumference.

Climbing stems represent efforts on the part of plants to economize material by utilizing the rigidity of neighbouring plants, and by reaching to the light on their shoulders. Here, as in aquatics, the rope type of stem is in evidence; it resembles a garden hose, offering great flexibility and conducting capacity, but without rigidity to support its own weight, much less that of the leaves and flowers which it bears. To secure support, the stem itself (or branches of it), the leaves, or the stipules (leafy projections on either side of the junction of leaf and stem), are used. Sometimes support is obtained by twining (compare Convolvulus, Grape-Vine, Vetch), sometimes by adherent discs (Virginia Creeper), or aerial roots (Ivy), often by mere scrambling, often aided by reflexed hooks on leaf and stem (Bramble, Cleavers). The mechanism by which twining is accomplished is of great interest. It is an effect of unequal growth of the different sides of the stem. If the unequal growth were confined to one side, the stem would eventually form a coil, or series of circles. But the region of greatest growth keeps shifting round the stem, with the result that the tip of the shoot describes a circle or ellipse, like the hand of a clock pointing successively in all directions. The stimulus is due, as in the case of the erect growth of ordinary stems (which usually display similar movements in a less degree) to gravity. Sometimes the movement, or nutation, is in the same direction as that of the hands of a clock (e.g., in the Hop); more frequently it is in the opposite direction, as of a clock-hand moving backwards. The result of this movement is that if the shoot encounters, say, an upright stem, it will lap round it in a spiral manner, and unless the said stem be quite smooth and unbranched, the twining shoot will be eventually supported by it. How effective the twining habit is as regards economy of building material may be seen from comparing the weight of the stem of a Hop with that of some tall herbaceous plant of the same altitude, and bearing an equal weight of leaves and flowers. The tendril-climbers are still more efficient, for they avoid the increased length of stem which arises from a twining habit. They grow straight up towards the light. Both the top of the growing shoot and the spreading tendrils which arise from it are continuously revolving in search of a support. When a tendril encounters one (such as a twig), the contact produces a stimulus which results in the tendril taking several close turns round the support. Nor does the action stop there, for usually the lower unattached portion of the tendril contracts into a spiral, drawing the stem closer to the support, and woody growth ensues, by which the tendril becomes exceedingly tough, often stronger than the stem itself.

One other point concerning climbers may be noted. Did they exhibit in a marked degree that bending towards the light which is characteristic of most plants, they would often defeat their own object, as they would grow away from possible supports. But they grow boldly up into an overhanging canopy, apparently confident of their power to ascend into the light and air which exist above. In the root-climbers, such as the Ivy, this bending away from the light is very marked; the stem presses closely to the bark or stone on which it creeps, probing every cranny, and the numerous rootlets by which it is attached are developed only on the dark side. But when the plant is old enough to flower, then branches devoid of roots grow out towards the light, so that the blossoms may be borne in the open, where they may be seen and visited by the numerous insects which, in their search for nectar, pollinate them.

In contrast to the extreme development in length found in the stems of climbing plants the extreme reduction of stem found in many plants of dry places may be referred to. The Crocus, for instance, has an abbreviated upright stem of which each year’s growth is distended for the storage of food: one year’s growth dies away as the next enlarges, so that the well-known bulb-like corm is produced. Compare the “roots”—really the stem—of Montbretia, in which the annual growths remain, the result being a knobby structure like a string of onions. In bulbs reduction in length is carried still farther, the stem forming a broad cone from the surface of which spring a number of modified leaves, forming fleshy scales swollen with food material; these surround and protect the bud, which when it grows produces green leaves and a terminal flower-shoot; growth is continued by axillary scale-leaved shoots situated among the scale leaves, which in due course themselves produce green leaves and flowers. These compact food-charged stems take up their position well below the ground, out of reach of intense heat or drought, and during the favourable season send up rapidly into the air their leaves and flowers, after which they remain dormant till the following year.