Xerophyte

“ It is here that the hygrophytes with buoyant seeds or fruits gather together, just as the xerophytes with similar seeds or fruits collect on the beaches. We have seen before that only a portion of the beach plants belong to the buoyant group, and the same applies to the plants at the edges of rivers and ponds. The plant-formation is no more homogeneous there than it is in the case of the strand-flora. ”

“ Sclerophyllous leaves are usually characterized by entire or sub-entire margins, a thick cuticle, small but rarely sunken stomata, a well-developed and close-set palisade tissue and a feeble system of air-spaces.
Hydro-xerophytes (“bog xerophytes”) .—Plants which live in wet, peaty soils, an which possess aeration channels and xerophilous leaves; e.g., Cladium Mariscus, Eriophorum angustifolium, Rubus Chamaemorus, and Vaccinium Vitis-Idaea. The term “oxylophyte” is open to the objection that some peaty waters are alkaline, and not acidic as the term implies. ”

“ Intense xerophytes and hydrophytes are much more fixed than mesophytes, though the intensity of the stimulus has doubtless as great an influence as its duration. Composites, labiates, grasses, orchids, etc., are less plastic than ranals, rosals, etc., but there are many exceptions to the apparent rule that fixity increases with taxonomic complexity. ”

“ Xerophytes and hydrophytes, on the other hand, exhibit marked diversity among themselves, a fact that makes it desirable to recognize subgroups, which correspond to fundamental differences of habitat or adaptation. It is hardly necessary to point out that these types are not sharply defined, or that a single plastic species may be so modified as to exhibit several of them. The extremes are always clearly defined, however, and they indicate the specific tendency of the adaptation shown by other members of the same group. ”

“ Phytoecology deals with all these, and includes the consideration of that struggle for life that plants are constantly waging, for environment determines that the forms best suited to a given set of conditions will survive. This struggle has been going on since the vegetable life of the earth began, and as a result certain prevailing conditions have brought about groups of plants found as a rule only where these conditions prevail. As water is a leading factor in plant growth, a classification is made upon this basis into the plants of the arid regions called xerophytes. ”

“ For accurate 35research, the chresard must of course be ascertained for each formation with respect to its peculiar plants and soil. The influence of the ecad in more or less determining the echard is also shown by Hedgcock, who found that floating plants wilt at 25 per cent, amphibious ones at 15–20 per cent, mesophytes at 6–12 per cent, and mesophytic xerophytes at 3–6 per cent. The echard is also somewhat higher for shade plants than for heliophytes. ”

“ The transpiring surface of a plant is reduced by decreasing the number of leaves, by reducing the size of each leaf, or by a change in its form. In so far as the stem is a leaf, a decrease in size or a change in shape brings about the same result. The final outcome of reduction in size or number is the complete loss of leaves, and more rarely, of the stem. Such marked decrease of leaf area is found only in intense xerophytes, though it occurs in all deciduous trees as a temporary adaptation. Changes in leaf form are nearly always accompanied by a decrease in size. ”

“ The general rule followed by most polydemics is that sun species will give rise to shade forms, and vice versa, and that xerophytes will produce forms of hydrophytic tendency, or the converse, when the areas concerned are not too remote, and the water or light differences are efficient, but not inhibitive. ”

“ A coarse gravel containing 15 per cent of water is practically saturated. The plants which grow upon it are mesophytes of a strong hydrophytic tendency, and they are able to use 141⁄2 or all but .5 out of the 15 per cent of water. In a compact clay, only 31⁄2 of the 15 per cent are available, and the plants growing in it are marked xerophytes. It is evident that a knowledge merely of the physical water-content is actually misleading in such cases, and this holds true of comparisons of any soils which differ considerably in texture. ”

“ Species which grow in exposed grassy or barren habitats are for the most part anemochores, while those that are found in the shelter of forests and thickets are usually zoochorous, though the taller trees and shrubs, being exposed to the upper air currents, are generally wind-distributed. There is then a fair degree of correspondence, inasmuch as most hydrophytes are hydrochorous, most hylophytes, zoochorous, and the majority of poophytes and xerophytes, anemochorous. ”

“ We do not aspire to a treatise on botany, but stop to consider only certain facts which are essential to this argument; a consideration of the higher plants will serve our purpose. Certain phytons are modified to become roots, which are the organs devoted to the absorption from the earth of the materials which are to be woven into the plant; other phytons become the stem for support; others the branches for expansion; others the leaves for respiration; others pistils and stamens for reproduction, while others become floral envelopes for their protection. ”

“ The modern flora of the earth admits of a grand twofold division into the Phænogamous, or flowering and seed-bearing plants, and the Cryptogamous, or flowerless and spore-bearing plants. In the former series, we have, first, those higher plants which start in life with two seed-leaves, and have stems with distinct bark, wood, and pith—the Exogens; secondly, those similar plants which begin life with one seed-leaf only, and have no distinction of bark, wood, and pith, in the stem—the Endogens ”

“ The stem of a rhizomatous plant may remain permanently underground, as is the case with all ferns except the tree ferns of the tropics. An annual stem is, however, usually sent to the surface and this may be a lateral branch from the main subterranean stem or it may be a continuation of the rhizome, in which case the next year’s rhizome will be a lateral branch and thus the whole rhizome will be made up of a number of distinct segments. ”

“ From their situation they are out of ordinary sight; but they will well repay examination. For the vegetation that is carried on under ground is hardly less varied or important than that above ground. All their forms may be referred to four principal kinds: namely, the Rhizoma (Rhizome) or Rootstock, the Tuber, the Corm or solid bulb, and the true Bulb.
Fig. 97. Rootstocks, or creeping subterranean branches, of the Peppermint.
104. The Rootstock, or Rhizoma, in its simplest form, is merely a creeping stem or branch growing beneath the surface of the soil, or partly covered by it. ”

“ Zeitung,’ 1861, p. 224) the purple fluid results from the metamorphosis of chlorophyll. Mr. Sorby examined the colouring matter with the spectroscope, and informs me that it consists of the commonest species of erythrophyll, “which is often met with in leaves with low vitality, and in parts, like the petioles, which carry on leaf-functions in a very imperfect manner. All that can be said, therefore, is that the hairs (or tentacles) are coloured like parts of a leaf which do not fulfil their proper office.” ”

“ In the modern world we divide all vegetation into two great series, that of the Flowering Plants (Phænogams) , which also produce true fruits and seeds, and that of the Flowerless Plants (Cryptogams) , which produce minute spores instead of seeds. The latter is in every respect the lower group. This lower series is again divisible into three classes—first and lowest, that of the Seaweeds, Moulds, and Lichens (Thallophytes) . Secondly, that of the Mosses and their allies (Anophytes) . Thirdly, that of the Ferns, Equisetums and Club-mosses (Acrogens) . ”

“ The roots of air-plants, such as certain orchids, draw their nourishment from the air.
The experiment of marking roots and stem has been already tried, but it should be repeated. Repetition of experiments is always desirable, as it fixes his conclusions in the pupil's mind. The stem grows by a succession of similar parts, phytomera, each part, or phyton, consisting of node, internode, and leaf. Thus it follows that stems must bear leaves. The marked stems of seedlings show greater growth towards the top of the growing phyton. It is only young stems that elongate throughout. ”

“ These abound in the cells of all common leaves and the green rind of plants, wherever exposed to the light. The green color is seen through the transparent skin of the leaf and the walls of the containing cells. Chlorophyll is essential to ordinary assimilation in plants: by its means, under the influence of sunlight, the plant converts crude sap into vegetable matter.
418. Far the largest part of all vegetable matter produced is that which goes to build up the plant's fabric or cellular structure, either directly or indirectly. ”

“ These “bordered” pits are very characteristic of the wood of all conifers.
The structure of the root is best studied in the seedling plant, or in a rootlet of an older one. The general plan of the root is much like that of the pteridophytes. The fibro-vascular bundle (Fig. 75, M, fb.) is of the so-called radial type, there being three xylem masses (x) alternating with as many phloem masses (ph.) in the root of the seedling. This regularity becomes destroyed as the root grows older by the formation of a cambium ring, something like that in the stem. ”

“ As the kernel swells with heat and moisture, its starches and albumens get oxidised and produce the motions and rearrangements of particles that result in the growth of a new plant. First a little head rises towards the sunlight and a little root pushes downward towards the moist soil beneath. The business of the root is to collect water for the circulating medium—the sap or blood of the plant—as well as a few mineral matters required for its stem and cells ”

“ V. Genus; VI. Species.
To illustrate: The white pine belongs to the highest great division (sub-kingdom) of the plant kingdom. The plants of this division all produce seeds, and hence are called “spermaphytes” (“seed plants”) . They may be divided into two groups (classes) , distinguished by certain peculiarities in the flowers and seeds. These are named respectively “gymnosperms” and “angiosperms,” and to the first our plant belongs. The gymnosperms may be further divided into several subordinate groups (orders) , one of which, the conifers, or cone-bearing evergreens, includes our plant. ”

“ They form large, dark green, felted masses, and are sometimes known as “green felts.” Some species grow also on the wet ground about springs. An examination of one of the masses shows it to be made up of closely matted, hair-like threads, each of which is an individual plant.
In transferring the plants to the slide for microscopic examination, they must be handled very carefully, as they are very easily injured. Each thread is a long tube, branching sometimes, but not divided into cells as in Spirogyra or Cladophora. ”

“ The leaf-traces arise from the bottom of each sinus; the leaf-bundles are mesarch, and consist exclusively of primary elements. The secondary xylem, x2, like that of the primary xylem, has a crenulate outer edge. The most interesting feature of the outer cortex is afforded by a tangential section which, in addition to the leaf-scars, cuts through a cone-scar showing a solid primary stele surrounded by the cortex of the cone-peduncle. ”

“ I am afraid it has been rather a difficult history to follow step by step, and yet it is perfectly clear when once we master the succession of growths. Starting from a spore, the thread-mass or protonema gives rise to the moss-stems forming the dense green carpet, then the green flowers on some of the leaf-stems give rise to a plant-egg, which roots itself in the stem, and grows into a perfect plant without leaves, bearing merely the urn in which fresh spores are formed, and so the round goes on from year to year. ”

“ On the other hand, there are the aërial roots of certain Orchids which, habitually green at their tips, continue green throughout their whole lengths when kept moist; which have become leaf-like not only by this development of chlorophyll, but also by the acquirement of stomata; and which do not bury themselves in the soil when they have the opportunity. ”

“ In the last case, if the plants used are from the same formation, the results will have almost the value of a field determination. When no definite habitat is the subject of investigation, an actual soil, and not an artificial mixture, should be used, and the plants employed should be mesophytes. The individual plants are grown from seeds in the proper soils, and are repotted sufficiently often to keep the roots away from the surface. The last transfer is made to a pot large enough to permit the plant to become full-grown without crowding the roots. ”

“ In order that they may perform their functions properly, however, it is essential that they be exposed to light; this is effected by their being in cells which are disposed in thin layers, such as we have seen the mesophyll of the leaf to be. In fact, the flat, thin, expanded form of the leaf is a direct adaptation to the end that these chlorophyll corpuscles shall be properly illuminated by the sunlight ”

“ The orchids are mostly small or moderate-sized plants, few of them being more than a metre or so in height. All of our native species, with the exception of a few from the extreme south, grow from fibrous roots or tubers, but many tropical orchids, as is well known, are “epiphytes”; that is, they grow upon the trunks and branches of trees. One genus, Vanilla, is a twining epiphyte; the fruit of this plant furnishes the vanilla of commerce. ”