Page:Encyclopædia Britannica, Ninth Edition, v. 17.djvu/881

ORCHIDS 817 tions, which reveals the existence of a complete series of in termediate forms constituting a regular gradation from the ordinary irregular gynandrous flower to regular flowers with six separate stamens such as we have taken as the type. What brings about what has brought about this ex traordinary series of changes, by virtue of which a flower typically as regular as a snowdrop becomes transmuted into the forms often more grotesque and extraordinary than can be found in any other group of plants 1 To the first part of the question the reply is that the present form has been inherited from generation to generation of orchids ; to the second part the answer most in accordance with the present state of knowledge is that these modifications are associated with the structure and habits of insects and their visits to the flowers. Cross fertilization, or the im pregnation 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 offspring, or one better able to adapt itself to the varying conditions under which it has to live. This cross fertilization is 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 germi nate on the stigma when its tubes travel down the style and their contents ultimately come in near apposition, per haps in actual confluence, with the " oosphere " or immature egg, which becomes in consequence fertilized, and there after gradually develops into a new plant. 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 wonder ful. 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. It would be quite impossible within the limits of this article to go into detail on this subject. All we can do is to give one or two illustrations, referring the reader desirous of fuller information to Darwin s Fertilization of Orchids. In the common orchids of British meadows, Orchis Morio, mascula (Shakespeare s long purples), tkc., the general struc ture of the flower is as we have described it. 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, usually 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 re maining 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 hollow lobes of 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 filled with viscid fluid, this is the stigma. 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 he v 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 lip and of the column, so that the insect, if of suffi cient size, while bending his head to insert his 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. Having attained his object the insect withdraws, taking with him the pollen-masses, and visits another flower. And now occurs another device or adaptation no less marvellous than those of which men tion has been made. The two anther- cases in an orchis are erect and nearly parallel the one to the other ; the pollen -masses within them are of course in like case, as may be thus represented ||, but immediately the pollen- masses are removed movements take place in 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 case of 0. pyramidalis, the two pollen- masses originally placed parallel || diverge from the ba>e 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 illustrations are comparatively simple; it would have been easy to select others of a more com plicated nature, but all evidently connected with the visits of insects and the cross fertilization of the flower. In some cases the form of the male flowers is so different from that of the female that before the different flowers had been found on the same spike, and before the facts of the case were fully known, they were taken to be representatives of distinct genera. The floral structure is so curious that perhaps less attention has been paid to the vegetative organs than the pecu liarities of their organization 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. 5), the tubers being partly radical partly bud- like in their character. There is often . ., j , FIG. 5. Tubercular roots a marked alternation in the production of Orcjlis mascu ia, a ter- of vegetative and flowering shoots re- restrial Orchid - spectively ; and, sometimes, from various circumstances, the flowering shoots are not produced for several years in suc cession, This fact will account for the profusion with which XVII. 103