363 SEXUALITY IN PLANTS. By MAXWELL T. MASTERS, M.D., F.R.S. [PLATE CIII.] IT T has long since become unnecessary to offer any argument or illustration in support of the doctrine of sexuality in plants. Among the so-called flowerless plants even, the evidence of the existence of distinct sexes is now so overwhelming that there are few who hesitate to believe in its occurrence, even in those cases where only one of the two sexes has up to this time been demonstrated. Linnæus, to whom, most of all, we owe the promulgation of the doctrine, though he was by no means the first to propound it, proceeded to establish it by inductive, circumstantial evidence. He brought forward comparatively little in the way of direct proof or absolute demonstration, even among flowering plants, while the actual existence of sexes in the so-called flowerless plants has been questioned up to within quite recent times. It is a singular fact, however, that the whole process of fertilisation has been, generally speaking, more thoroughly and completely demonstrated among the so-called Cryptograms, long considered as a sexual, than among the more highly developed flowering plants. It is curious also to remark the different views now taken as to the process from those which were held originally. As soon as it was clearly perceived that the stamens and the pistils, or their contents, were the essential agents in the process of fertilisation, it was naturally surmised that the pistil of every given flower was fertilised by the pollen of the same flower. Under this supposition many curious contrivances which were observed were at once set down as so many aids and promoters of selffertilisation. Sprengel was one of the first to show the fallacies of these observations, and to demonstrate the frequent existence of cross-fertilisation, whether effected by the agency of winds or by pollen-carrying insects. Vaucher added other illustrations, but it was not till the publication and experimental researches of Darwin that the subject received the attention it merited. Darwin's classical paper on Primula, in the " Journal of the Linnean Society," followed up by similar memoirs on Lythrum, Linum, &c., stirred up a host of observers. Large numbers of new facts were recorded, all confirming the idea that while self-fertilisation is not impossible, and is indeed in some few cases inevitable, yet in the majority of instances some provision for cross-fertilisation is afforded, if not constantly, at least occasionally. Hildebrand, Delpino, Alfred Bennett, and others, have correlated these facts, and, making allowance for exceptional instances, they one and all confirm the views expressed by Darwin. The observations and experiments of these gentlemen are so well known, and the records of them are so accessible, that we do not propose now to occupy space by making further allusions to them. Suffice it merely to say, that these observations have reference to the facilities offered by various structural modifications for favouring cross-fertilisation and preventing self impregnation or vice versa, and they all tend to show the advantage that accrues from an occasional cross. Our object in the present communication has reference to another department of the subject; and it is one we think it desirable to call attention to, as it appears not to receive so much attention as its importance demands. We allude particularly to the circumstances promoting the development of pollen-forming or ovule-bearing flowers respectively. Before proceeding further we ought to explain that in using the expressions male, female, or hermaphrodite flowers, we do so, unless otherwise stated, with reference to structural conditions rather than to physiological office. The term "bisexual" is preferable to that of hermaphrodite, as not implying any physiological distinction. For our present purpose, then, a flower with stamens and pistils perfectly formed is bisexual or hermaphrodite, even though its pistil be not fertilised by its own pollen, but by that derived from some other source. For convenience sake, we take a bisexual flower as our starting-point; and we propose to allude to various not infrequent changes observed in flowers of that description, in consequence of which their sexual organisation becomes more or less materially modified. A plant usually producing flowers, bisexual or hermaphrodite as to structure, may bear flowers of one sex only by the simple arrest of growth. Thus, if the stamens of any given flower be arrested in their development, the blossom becomes female, as happens not unfrequently in some buttercups, lesser celandine (Ranunculus), and others. Conversely, if the pistil be not developed the adult flower will be male, as happens in many Umbellifers and Caroyphylleæ. The exact opposite of this change occurs when flowers usually of one sex become bisexual by the development of stamens or of pistils, as the case may be. Instances of this kind are common in almost all normally unisexual flowers. If the unisexual condition of an ordinary bisexual plant be considered from a structural point of view as an arrest of development, the present instance must be attributed to an exaltation of that process. There is another way in which an ordinarily one-sexed flower may become two-sexed, and that is by the more or less perfect change of stamens into pistils, or of pistils into stamens. This is sometimes the result of a substitution of one part for another, but in other cases of an actual permutation. A stamen becomes converted at a certain stage of its growth into a pistil, or vice versa. Such changes are by no means uncommon in plants. We may thus have a stamen assuming the guise of a pistil, a pistil endowed with all the attributes of a stamen; we may even have pollen formed within the tissues of the ovule itself, as has been seen in a passion-flower and in a rose. A more complete hermaphroditism can hardly be conceived. So far we have been dealing with individual flowers, but analogous changes occur throughout the whole organism. Thus many plants, under ordinary circumstances, produce unisexual flowers, male and female, on the same individual. Such are the plants called monacious. Now it sometimes happens that plants of this character become entirely unisexual by the development of male or of female flowers, only to the exclusion of the other. Such occurrences are not uncommon in mulberries and walnuts. The converse of this is, when a plant ordinarily producing flowers of different sexes on different individuals (diæcious) forms flowers of both sexes on the same plant, becomes, in other words, monoecious. This occurs occasionally in the hop. These changes in the structural condition of the flower, variously modified and combined in different cases, constitute all the changes in the sexual organisation of the flower which concern us at present. May we not say of these puzzling transformations what Horace said of a girlish-looking youth Mire sagaces falleret hospites |