Page:EB1911 - Volume 20.djvu/197

Fig. 5. — Pollenmasses

of an Orchid,

with their caudicles

c and

gland g.

nearest to the supporting stem, becomes in course of growth turned to the anterior or lower part of the flower nearest to the bract, from whose axil it arises. Other common modifications arise from the union of certain parts of the perianth to each other, and from the varied and often very remarkable outgrowths from the lip. These modifications are associated with the structure and habits of insects and their visits to the flowers.

Cross fertilization, or the impregnation of any given flower by pollen from another flower of the same species on the same or on another plant, has been proved to be of great advantage to the plant by securing a more numerous or a more robust off.spring, or one better able to adapt itself to the varying conditions under which it has to live. This common cross fertilization is often effected by the agency of insects. They are attracted to the flower by its colour or its perfume; they seek, collect or feed on its honey, and while so doing they remove the pollen from the anther and convey it to another flower, there to germinate on the stigma when its tubes travel down the style to the ovary where their contents ultimately fuse with the " oosphere " or immature egg, which becomes in consequence fertilized, and forms a seed which afterwards develops into a new plant (see article Angiosperms). To facilitate the operations of such insects, by compelling them to move in certain lines so as to secure the due removal of the pollen and its subsequent deposit on the right place, the form of the flower and the conformation of its several parts are modified in ways as varied as they are wonderful. Other insects visit the flower with more questionable result. For them the pollen is an attraction as food, or some other part of the flower offers an inducement to them for a like object. Such visitors are clearly prejudicial to the flower, and so we meet with arrangements which are calculated to repel the intruders, or at least to force them to enter the flower in such a way as not to effect mischief. See Darwin's Fertilization of Orchids and similar works.

In the common orchids of British meadows. Orchis Morio, mascula (Shakespeare's long purples), &c., the general structure of the flower is as we have described it (figs. 2, 3). In addition there is in this particular genus, as indeed in many others, a long tubular spur or horn projecting downwards from the back of the lip, whose office it is to secrete and store a honeyed juice; the forepart of the lip forms an expanded plate, usuaUy larger and more brightly coloured than the other parts of the flower, and with hairs or ridges and spots of various kinds according to the species. The remaining parts of the perianth are very much smaller, and commonly are so arranged as to form a hood overarching the " column. " This column stands up from the base of the flower, almost at right angles to the lip, and it bears at the top an anther, in the two hollowlobesof which are concealed the two pollen-masses, each with its caudicle terminating below in a roundish gland, concealed at first in the pouch-like rostellum at the front of the column. Below the anther the surface of the column in front is hollowed out into a greenish depression covered with viscid fluid — this is the two united stigmas. The other parts of the flower need not detain us. Such being in general terms the mechanism of the flower of a common orchis, let us now see how it acts. A bee, we will assume, attracted by the colour and perfume of the flower, alights on that part of it which is the first to attract its attention — the lip. There, guided by the hairs or ridges before-mentioned, it is led to the orifice of the spur with its store of honeyed juice. The position of this orifice, as we have seen, is at the base of the hp and of the column, so that the insect, if of sufficient size, whOe bending its head to insert the proboscis into the spur, almost of necessity displaces the pollen-masses. Liberated from the anthers, these adhere to the head or back of the insect by means of the sticky gland at the bottom of the caudicle (fig. 4). Having attained its object the insect withdraws, taking the pollen-masses, and visits another flower. And now occurs another device or adaptation no

less marvellous than those of which mention has been made. The two anther-cases in an orchis are erect and nearly parallel the one to the other; the poUcn-masses within them are of course in like case, as may be thus represented II, but immediately the pollen-masses are removed movements take place at the base of the caudicle so as to effect the bending of this stalk and the placing the pollen-mass in a more or less horizontal position, thus =, or, as in the ca.se of O. pyramidal is, the two pollen-masses originally placed paraUel || diverge from the base like the letter V. The movements of the pollen-masses may readily be seen with the naked eye by thrusting the point of a needle into the base of the anther, when the disks adhere to the needle as they would do to the antenna of an insect, and may be withdrawn. Sometimes the lip is mobile and even sensitive to impressions, as are also certain processes of the column. In such cases the contact of an insect or other body with those processes is sufficient to liberate the pollen often with elastic force, even when the anther itself is not touched. In other orchids movements take place in different ways and in other directions. The object of these movements will be appreciated when it is remembered that, if the pollen-masses retained the original direction they had in the anther in which they were formed, they would, when transported by the insect to another flower, merely come in contact with the anther of that flower, where of course they would be of no use; but, owing to the divergences and flexions above alluded to, the pollen-masses come to be so placed that, when transplanted to another flower of the same species, they come in contact with the stigma and so effect the fertilization of that flower. These iUustrations are comparatively simple; it would have been easy to select others of a more complicated nature, but all evidently connected with the visits of insects and the cross fertilization of the flower. In some cases, as in Catasctum, male flowers are produced so different from the female that before the different flowers had been found on the same pike, and before the facts of the case were fully known, they were taken to be representatives of distinct genera.

The fruit is a capsule splitting generally by three longitudinal slits forming valves which remain united above and below. The seeds are minute and innumerable; they contain a small rudimentary embryo surrounded by a thin loose membranous coat, and are scattered by means of hygroscopic hairs on the inside of the valves which by their movements jerk out the seeds. The floral structure is so curious that perhaps less attention has been paid to the vegetative organs than the peculiarities of their organisation demand. We can only allude to some of these points. The orchids of British fields are all of terrestrial habit, and their roots are mostly tuberous (fig. 6), the tubers being partly radical partly budlike in their character. There is often a marked alternation in the production of vegetative and

flowering shoots respectively; and, sometimes, from various circumstances, the

flowering shoots are not produced for

several years in succession. This fact will account for the profusion with which some orchids, like the common bee orchis for instance, are found in some seasons and their scarcity in others. Tropical orchids are mostly epiphytal — that is, they grow upon trees without deriving nourishment from them. They are frequently provided with " pseudo-bulbs, " large solid swellings of the stem, in the tissues of which water and nutritive materials are stored. They derive this moisture from the air by means of aerial roots, developed from the stem and bearing an outer spongy structure, or velamcn, consisting of empty cells kept open by spiral thickenings in the wall; this sponge-like tissue absorbs dew and rain and condenses the moisture of the air and passes it on to the internal tissues.

Fig. 6. — Tubercular

roots of Orchis

mascula, a terrestrial

Orchid.