New Zealand Plants and their Story
Origin of Special Forest Plants
Origin of Special Forest Plants.
The forest also tells us a good deal about the evolution of the wonderful adaptations of certain plants to the conditions it provides. On walking through its interior one cannot fail to notice the subdued light, which is so much less than in the open. Above all things, most plants require sunlight. Without this they cannot manufacture in their leaf laboratories their necessary food from the carbonic acid of the air. In a forest, then, there must be a struggle for the sunlight. The tall trees meet the difficulty by raising their tops high into the heavens. But with the smaller plants it is another matter, and these must either become attuned to a minimum of light, or make some special effort to get their fair share. Consequently, we find a spindling habit of growth in many young forest-trees—long, straight, thin stems, and few lateral branches; "drawn up to the light" is the gardener's phrase.
Carry out this idea a little further, and you have certain plants putting out long shoots, which, too weak to stand alone, lean against other trees for support. Go a little further still, and such long shoots develop certain organs to assist them to cling to the supporting tree. So, by slow degrees, modification after modification arises for the end in view, until the wonderful family of lianes or climbing-plants is evolved, whose roots can enjoy the cool and rich soil of the forest-floor, but whose crowns dispute with the tree-tops for the light of heaven, and under its influence bring forth their flowers, ripen their fruits, and manufacture stores of food within their green leaves.
Lianes may be conveniently divided into scramblers, root climbers, twiners, and tendril climbers, names which speak for themselves. Fuchsia Colensoi, a much more slender plant than the tree-fuchsia page 30(F. excorticata), offers a transition to the scrambling habit, being frequently merely a shrub, and at other times a true liane, its thin shoots being thrust amongst the branches of another tree for their support. Here there is no special differentiation of climbing-organs; but in the various species of Rubus it is different. On their leaf-stalks and midribs these have developed special curved hooks for climbing purposes, which grip so tenaciously whatever they touch that they have earned for these plants the sarcastic term of "lawyer." Frequently the leaf-blades are much reduced in size, and the midribs are elongated, so that the leaf is changed in function, and has become a special climbing-apparatus. In New Zealand there are several species of Rubus, which differ considerably in shape of leaf, size of flower, and colour of fruit, the commonest and the one with the largest leaves and most showy flowers being R. australis. One of the commonest root climbers, which with its leathery, green, sword-like leaves much affects the physiognomy of northern forests, is the kiekie (Freycinetia Banksii), whose fleshy bracts, called "tawhara" by the Maoris, are sweet and edible. The roots fasten the plant very firmly to the support, being given off at right angles or thereabouts to the stiff climbing-stem, and, passing right round the support if slender, finally put forth many rootlets, which are parallel, or nearly so, to the main roots, and close together.
Fig. 11.—The Liane, the Supplejack (Rhipogonum scandens), growing as a member of a taxad forest.
[Photo, L. Cockayne.
The well-known supplejack (Rhipogonum scandens), a plant of the lily family, forms close entanglements in most lowland forests (fig. 11) Originally many of these stems have wound round young trees, which page 33have been strangled to death, while others have broken away from the branch to which they had clung. The two species of Muehlenbeckia, relatives of the common dock, are also twining-plants. They are easily recognised by their soft, green, abundant leaves, and when in fruit by the small black nuts seated on a fleshy and almost transparent cup. Very frequently, as bush boys and girls well know, their rope-like stems hang swaying from the forest-roof, the original support long vanished. Parsonsia heterophylla, a pretty plant producing abundance of small sweet-scented flowers, is another very common twining-liane. It occurs especially on the forest-outskirts, or where the bush has been partially cleared. It and its near relative, P. capsularis, may be recognised by the curious long green fruit, something like a kidney-bean in outward appearance. It is especially remarkable for the diversity of forms assumed by its leaves. These may be arranged into three series—viz., small round, long narrow, and finally moderately broad and of an oblong type. Between the small round and the long narrow are all kinds of transitional forms. One variety of the related P. capsularis never reaches the final adult stage, but produces flowers while in the narrow-leaved condition, and so it may perhaps be considered a fixed juvenile form of Parsonsia heterophylla.
The mange-mange (Lygodium articulatum) is a beautiful climbingfern, whose masses of tough slender stems wound round one another make a substitute for a wire-wove mattress by no means to be despised. The leaf of an ordinary fern consists of a stalk and blade, the continuation of the former being called the midrib. The blade may be divided or undivided; in the former case the divisions may be little leaves, each with its own stalk. In nearly all cases the leaf continues to increase in length for a certain time, when its growth is concluded. There is usually no further increase year after year. But the remarkable fern we are considering (Lygodium) is regulated by no such rule, for its midribs may continue to grow until the leaf is so long as to reach the tops of tall trees. The midrib thus has become a climbing organ, and a leaf many yards in length is different altogether from what one imagines a leaf to be. At regular intervals lateral leaflets, which are also capable, of great extension, are given off from the midrib, one at a time, and distant from each other about 4 in., each being furnished with a very short stalk. Two quite different kinds of leaflets may be page 34noted—those which bear spores,* and those which function as ordinary leaves—but between the two are all kinds of transitional stages, very interesting to observe.
Those beautiful flowering-plants, the clematises, are tendril climbers, the tendrils being modified leaf-stalks. Clematis indivisa is the large white-flowered species; C. hexasepala has also white but smaller flowers; C. Colensoi produces masses of yellow flowers in the spring. It is especially abundant in the Wellington Province. C. afoliata is a curious form which looks rather like a mass of rushes. It has few or no true leaves; but they would be a harm rather than a benefit, for it grows in extremely dry places. All the New Zealand species of Clematis have male and female flowers on separate plants, the male being much the more showy.
The New Zealand passion-flower (Tetrapathaea australis) is another tendril climber. In autumn its orange or red fruits, containing numerous black seeds, are very showy. It is not found everywhere, and does not go farther south than Banks Peninsula.
All the lianes are worthy of the closest study, and not the least interesting point is to observe the differences between the climbing and non-climbing shoots. Also, it is remarkable how certain species, such as some of the lawyers and Metrosideros scandens, are lianes under one set of conditions and virtually shrubs under another. It is interesting, too, to grow this class, of plants from seed, and to observe how the climbing habit is not shown at all, or very little, by the early seedling (fig. 12).
Fig. 12.—On left, Seedling of a finally much-branched Drought-resisting Shrub. On right, Seedling of a Climbing-plant.
[Photo, L. Cockayne.
Fig. 13.—Roots looking like climbing stems descending down trunk of a Rimu (Dacrydium cupressinum) coming from an epiphytic Broadleaf (Griselinia littoralis).
Lands Department.] [Photo, L. Cockayne.
Though many plants are eager to get into the fresh air and sunlight, others are, the reverse, and have developed different adaptations in accord with other aspirations. The interior of a thick forest has an atmosphere charged with vapour not altogether unlike that of a glasshouse. Plants living under such conditions are subject to much the same environment as submerged water-plants, and have developed similar leaves, which are so thin as to be able to absorb any water which may fall upon their surfaces. Such, amongst others, are the filmy ferns (species of Hymenophyllum and Trichomanes), the beautiful crape-fern Todaea superba, and its relative Todaea hymeno phylloides. Plants like these can exist only in a moist atmosphere; the full rays of the sun or a dry atmosphere cause them to shrivel up, and they soon die when removed from their forest home. Many mosses and liverworts also belong to this category, and mimic in their forms the smaller ferns, to which, of course, they bear no relationship.
* A spore is any single cell that becomes free from the parent plant and is capable of developing into a new individual. The spores of ferns are contained in spore-cases, and groups of these make the dots or round patches on the under-surfaces of some of the leaves of ferns.