Part 52

6,000 (10,000?) species. The majority live in the Tropics and occur, especially, as epiphytes on trees or in the crevices of rocks, to which they are attached by aerial roots. These aerial roots, like those of Araceæ, are covered by several layers of spirally-thickened cells (tracheides) which contain air and form the velamen--an apparatus to absorb moisture from the air. The roots have a white appearance when the cells are filled with air, which changes to a greenish hue when they are filled with water, the chlorophyll then shining through. They generally have horizontal rhizomes; the ascending shoots, which bear the foliage-leaves, may vary, but they very often swell and assume the form of a tuber, which persists for several years fresh and green after the leaves have fallen off (Fig. 321). Vanilla is an exception (see above). Our Orchids are all terrestrial (or marsh-plants); the largest number of species is found in calcareous soils.

POLLINATION takes place principally by means of insects, but self-pollination occurs in some. The lip serves as a landing-stage for the insect visitors, which, on sucking the honey, cause the adhesive discs, with the pollinia attached to them, to adhere to their bodies (generally to the probosces) and so carry them away to other flowers. In some species parts of the flower are sensitive or irritable, which has some connection with the pollination. Without doubt there are a great many biological differences which are closely connected with the infinite multiplicity of forms; Darwin (1862) has already shown an enormous variety, never even dreamt of before, in the European species. The genus Catasetum has ♂-♀-and ☿-plants with flowers of such different appearances that they have been classed in various genera (Myanthus, Monacanthus). Platanthera is pollinated by hawk-moths; Ophrys, by flies; Epipactis latifolia, by wasps; Orchis, by bees, especially humble-bees, etc.

The DISTRIBUTION OF SEEDS is effected by the wind, the seeds being so exceedingly small and light. Many species moreover have peculiar, elater-like, fine, hygroscopic hairs in the ovary, which eject the seeds in a manner similar to the elaters of the Liverworts.

The USES are few, mostly as ornamental plants in conservatories. The tubers of several Orchis-species are OFFICINAL; they contain starch and mucilage and are used us “salep.” The fruits of Vanilla planifolia are used as condiments and differ from other Orchid-fruits in being rather fleshy and in dehiscing irregularly; the seeds are very small, shining and black.

Class II. =Dicotyledones.=

In this class THE EMBRYO has 2 seed-leaves, a rule from which there are few exceptions (e.g. Ficaria, Cyclamen, Pinguicula, certain species of Corydalis, with only 1; and a few, mostly parasitic forms, e.g. Monotropa, Orobanche, Pyrola, entirely without cotyledons). On germination the cotyledons nearly always raise themselves above the ground as green, assimilating leaves and are then termed aerial or epigean, in contradistinction to the underground or hypogean which are always buried. The structure of the seed varies (endospermous or exendospermous); the embryo may be straight or curved. In many instances the primary root grows as a vigorous tap-root, with weaker branches arising acropetally (in annuals, biennials, many perennials, especially woody plants); but in a large number of herbaceous perennials, which have rhizomes, the root behaves very much as in the Monocotyledons. The roots generally increase in thickness by means of a cambium.

THE STEM, when seen in transverse section, has its vascular bundles arranged in a ring; in reality, however, they form a kind of cylindrical network in the stem; the bundles are open, and thickening takes place by means of a cambium; annual rings are formed in the perennial stems. There is a rich and very varied form of branching. The two first leaves of a shoot (fore-leaves) are placed nearly always to the right and to the left; the same arrangement is found in the two first leaves developed on the flower-stalk, and these are, as a rule, the only two; they are found below the calyx and are usually termed the “bracteoles.” It has become customary to indicate the bracteoles by the letters α and β, according to their sequence of growth, and in that sense these letters will be employed in the following diagrams.

THE ARRANGEMENT OF THE LEAVES varies very much; there is also a great variety of shapes in the leaves and their venation, but the linear leaves, with parallel venation, so frequent in the Monocotyledons, are seldom met with, as also the large sheaths (though the sheath is well developed in the Umbelliferous plants); stipules occur much more frequently.

THE FLOWER is most commonly cyclic, but acyclic or hemicyclic forms also occur. The type which may be taken as a basis consists in the majority of instances, as in the Monocotyledons, of 5 whorls, of which the 4 outer ones (calyx, corolla, and the 2 whorls of stamens) are most frequently 4 or 5 in number and placed in regular alternation, whilst the innermost one (the carpels) has generally fewer members, probably on account of space (Figs. 360, 361, 421, 429, 487, etc.). Trimerous (Figs. 384, 387, etc.) flowers, or those in which the members of the flower are in threes or a multiple of three, also occur, as well as dimerous flowers; other numbers are rare. It is of the greatest importance in connection with the relative position of the members of the flower to the axis and bract (orientation), whether the bracteoles are typically present (even though they may not be developed), or are typically absent. If there are 2 bracteoles present, then their position in a pentamerous flower is often as follows: the first sepal turns obliquely forward, the second is posterior and median, the third obliquely forward, the fourth and fifth obliquely backward; quincuncial æstivation is often found in these buds (Figs. 360, 429, 471, 475, 584). The first and third leaves, in the following chapters, are most frequently alluded to as the “anterior,” the fourth and fifth as the “lateral” leaves. The reversed arrangement, with the median sepal in the front, occurs for instance in Papilionaceæ (Fig. 511), Lobeliaceæ (Fig. 594), Rhodoracecæ. If any bracteoles are present below a tetramerous flower, the relation is generally that 2 sepals (the first ones) stand in the median plane, the two next ones transversely (Fig. 393), and the corolla then adopts a diagonal position (Fig. 397); but a diagonal position of the calyx generally shows that the flower is not, strictly speaking, tetramerous, as in Plantago (Fig. 567), Veronica (Fig. 559 C) and others.

If the bracteoles are not typically present, then the position of the sepals is changed accordingly, and the two outer sepals endeavour to assume the position which the bracteoles would otherwise have occupied, e.g. in Primula (Fig. 547). Other positions are also found when the number of bracteoles is more or less than two.

The leaves which follow the sepals occupy definite positions with regard to them, which we may consider later. An arrangement must, however, be mentioned here; when the flower is “diplostemonous” that is, has two whorls of stamens (thus, Sn, Pn, An + n), these may be arranged in two ways. Either the first-formed whorl of stamens, which are termed the “calyx-stamens,” stands directly in front of the sepals (that is “episepalous”), and is the outermost whorl, and in this case a regular alternation takes place between sepals, petals and the two whorls of stamens, which is also continued into the carpels if their number is the same as that of the other whorls: the carpels are then placed opposite the sepals (Fig. 278) and the flower is isomerous and Gn should be added to the formula above. Or, the calyx-stamens form the innermost whorl, and the corolla-stamens, which are subsequently formed (“epipetalous” stamens), stand outside these (Figs. 360, 429); if the number of carpels is the same as that of the preceding whorls, they are often placed right in front of the petals and the corolla-stamens. The first-mentioned arrangement is termed Diplostemonous, and the second Obdiplostemonous. ~Both arrangements may be found in one and the same order, e.g. Caryophyllaceæ. The size and relation of the members of the flowers, and also the contact with other members in the early stages of their development, play an important part in determining the arrangement.~

The great number of structural arrangements found in this enormously large class, may, as is the case in the Monocotyledons, be further varied by suppression and division of certain leaves (especially the stamens). Instances of this will occur in the following (Figs. 559, 568.--426, 441, 445, etc.).

The Dicotyledons were formerly divided into 3 sub-classes: Apetalæ (those without corolla), Sympetalæ or Gamopetalæ (those with the petals united), and Choripetalæ or Polypetalæ (the petals not united). This division has now been abandoned because it has been proved that the Apetalæ were merely reduced or incomplete forms of the Choripetalæ, and they have therefore been distributed among the various families of the latter sub-class.

With regard to the Sympetalæ (or Gamopetalæ) it may be stated that they form to a very great extent a closely connected and natural group, having in common not only the character that the corolla is gamopetalous and the stamens united with it (this being also found in the Choripetalæ), but also a great many others (such as persistent calyx, cyclic flowers with the formula S5, P5, A5 and as a rule G2, the two carpels being united to form the ovary; seeds with a thick integument and a very small nucellus). They are therefore considered as an independent sub-class, and must be placed at the close of the system of classification as the forms which presumably have arisen the latest. In the future systems of classification this arrangement will very probably be changed, and the first families of the Sympetalæ, the Bicornes and others will for instance be to a certain extent united with the families or orders of the Choripetalæ. The Sympetalæ may certainly be considered as the youngest types, the strongly pronounced metamorphosis supporting this theory, as also the formation of the integument of the ovule, the one thick integument being undoubtedly derived from the coalescence of two--a holochlamydeous ovule, etc.

The Apetalæ and Choripetalæ are united into one sub-class. The leaves of the perianth in this case are, as a rule, free from each other, the structure of the flowers presents many differences, and the ovules have as a rule 2 integuments and a large nucellus. Considerable uncertainty still prevails regarding the arrangement and the relationship of the individual families of the Choripetalæ, and some of the following families are hardly quite natural; but the best arrangement arrived at so far has been adopted here.

At the beginning of the book a review of the orders of the Dicotyledons will be found.

Sub-Class 1. =Choripetalæ. Petals free.=

Family 1. =Salicifloræ.=

Trees and shrubs, which, in the structure of the vegetative shoot and the catkin-like inflorescences, resemble the Quercifloræ, but the structure of the flower differs so much from them, that the only order brought under this heading--Salicaceæ--well deserves to be separated and to form a family of its own, the nearest relatives of which are still doubtful. ~As Juglandaceæ and Myricaceæ also deserve to be placed in a special family, the name Amentaceæ (Catkin-bearers), hitherto applied to all of these plants, cannot be retained as the name of a family.~

There is only one order.

Order. =Salicaceæ= (=Willows=). Trees with simple, scattered, stipulate leaves. Diœcious. The flowers are arranged in simple inflorescences (spikes or racemes) which are termed catkins, and which fall off as a whole after flowering (♂) or after the ripening of the fruit (♀) (Fig. 322). The perianth is very imperfect or wanting, particularly in Salix (Fig. 323 o); the ♂-flower with 2–several stamens and without any trace of a carpel (a, b, c): the ♀-flower has a free bicarpellate ovary, unilocular, and formed from 2 lateral carpels with 2 parietal (median) placentæ and generally ∞ ovules; the style divides into two stigmas (d, e, f). The fruit is a two-valved capsule and the very small seeds bear a tuft of hairs at the base. Endosperm absent.--~The catkins are situated on dwarf-branches, which in some species often develop before the leaves and bear at their base only scale-leaves; in others foliage-leaves are borne beneath the catkins. The vegetative bud commences with 2 bud-scales which are united on the anterior side into a scale. The capsule opens by the dorsal suture. The seed-hairs spring from the funicle.~

Salix (Willow) has short-stalked, most frequently lanceolate leaves and erect catkins with undivided bracts (Fig. 322). The flowers are naked; 1 (o in a-f) or 2 yellowish glands situated in the median line. In the ♂-flower generally two stamens, situated laterally like the carpels in the ♀-flower. ~Various forms are seen in Fig. 323.--The terminal bud of the branches often aborts regularly, the uppermost lateral bud taking its place.~

Populus (Aspen, Poplar) has long-stalked, more or less round or cordate leaves with drawn-out apex; catkin pendulous; lobed bracts; perianth cup-like with oblique edge; stamens usually numerous; stigmas often divided.--~P. tremula (Aspen) has received its name from the tremor of the leaves: cf. “to shake like an aspen leaf.”~

POLLINATION. The Poplars are wind-pollinated. The Willows have sticky pollen and are pollinated by insects. The catkins of the Willows, especially the ♂, are more conspicuous, from the numerous, closely-packed, yellow flowers, rich in honey and pollen. The catkins often appear before the foliage and so are much more easily seen, whilst at this time of the year the number of competing honey-flowers is smaller, and the insect visits consequently more numerous. On many catkins of the Willow the flowers open earliest on the side which is turned towards the sun and in descending order, i.e. the upper flowers develop before the lower ones. Hybrids frequently appear.

There are about 180 species existing in the northern, cold and temperate latitudes. Some in the Polar regions are scarcely more than an inch in height, and have a creeping rhizome (Salix herbacea, polaris, reticulata). Fossil forms are found in the Tertiary and perhaps also in the Upper Cretaceous.

USES. Principally for ornamental trees, as they grow very quickly and are easily propagated by cuttings, S. babylonica, Weeping Willow; S. purpurea; Populus alba, Silver Poplar; P. pyramidalis, Pyramid Poplar--a form of P. nigra; P. monilifera, Canadian Poplar. The wood is very poor and little used; the branches of many Willows are cultivated for basket-making, etc. The wood of the Aspen is used for matches. The bark contains tannin and, in many Willows, a very bitter extract, Salicin (S. pentandra, fragilis). Salicylic acid (officinal) is obtained from Salix. Balsam is extracted from the buds of many Poplars, especially when the leaves are shooting.

Family 2. =Casuarinifloræ.=

Trees with verticillate, scale-like leaves forming sheaths at the nodes. Monœcious. Flowers unisexual. ♂-flowers in catkins; ♀ in short spikes. Pollen-tube entering the ovule at the chalaza, and not through the micropyle. Ovary 1-seeded, unilocular. Carpels uniting into a multiple fruit. Only one order.