ANGIOSPERMS
433
These leaves may be together on the same torus close over the sporophylls and open again, more than once, {hermaphrodite jiower), or on separate ones {unisexual in response to various external stimuli. Their colours, flower). The stamen has typically a petiolar portion primarily parasolar, appear to be attractive to birds and '(jilament), and a laminar portion {anther), and within the insects, and these by their visits aid in the pollination of latter the pollen-sacs, of which there are commonly four, the flower, and have been agents in the intensification of each containing many pollen-grains, are embedded. The the colours through selection. The leaves of the flowercarpel is a leaf folded upon itself to form an ovary investing envelopes vary in every way more than do the sporothe ovules. These arise, each with its embryo-sac, from the phylls, and show frequently extreme specialization in their tissue of the placenta placed either upon the carpel (foliar) forms, which in many cases appear to be correlated or upon the torus (axial) which projects into the cavity of with the visits of birds and insects. Another accessory the ovary; in epigynous flowers the torus shares more or feature present in many flowers, and especially Nectaries_ less in the formation of the wall of the cavity. The apex in those with colour, is the secretion of honey, of the carpel is more or less prolonged as the style upon with the associated odour, from nectaries. These nectaries, which a special papillar or hairy secreting germinating often termed nuptial, to distinguish them from similar surface of varying extent, the stigma, is developed. Upon ones which occur upon parts of the plant away from the this stigmatic surface the pollen-grains which are carried flower (extranuptial), may arise upon the torus independto it (pollination) from their mother pollen-sacs, continue ently of the flower-leaves, or may be produced by any one their germination, and then through the pollen-tube the of these leaves. The secretion is usually discharged from male gametes they produce are conveyed to the female the nectaries, but in some cases is retained, and can only gamete enclosed in the megaspore. The anther-wall be secured by puncture. Odour appears to be the most enclosing the pollen-sacs is commonly no more than an potent attraction t*o birds and insects possessed by flowers. The flower has primarily a radial construction with a spiral or epidermis in the greater part of its extent, and usually cyclic (or these combined) disposition of the flower-leaves ; the contains parasolar colouring substances which protect the terminal flower of an inflorescence is always radial. Lateral developing pollen-grains. It forms with the walls of the flowers may be radial, but are frequently dor si ventral, a conpollen-sacs a hygroscopic mechanism, through which it struction which, primarily an adaptation to physical needs, has intensified by insect-agency. The relative number and is ruptured in definite areas for the escape of the been positions of the different kinds of flower-leaves are characteristic pollen-grains at the period when these are mature. The features, and in lateral flowers have a definite relationship to the ovarian wall is green, or possesses at least some chloro- mother-axis of the flower, as well as to the prophylls—the common phyll and frequently meristem, and thereby is. fitted trimery of Monocotyledones showing typically an anterior sepal relation to the one posterior prophyll, whilst in the pentamery for the subsequent developments through which it in and dimery of Dicotyledones there is usually a posterior sepal becomes the fruit, enclosing for a longer or shorter with a pair of lateral prophylls. All these relationships (and period the seeds into which the ovules develop. . The they are of great variety) of the flower-leaves are of considerable number, form, size, colour, cohesion, adhesion, branching of value in the estimation of the genetic relationships of the families AnGosperms. The formation of flower concludes the growth in the sporophylls give manifold variety of flower-structure, but of length of the axis to which it belongs. If this be the primary, or the features in the sporophylls which are of significance relatively primary axis the plant is monaxial; the majority ot from the phyletic standpoint, and by which the Angio- plants are, however, pluriaxial, the flowers belonging to axes of sperms as a whole are distinguished from the Gymnosperms higher degree. In both cases there is a tendency to aggregation of (l) the greater protection afforded to the ovules by the flowers in inflorescences, whereby small flowers become more conare spicuous and the function of the flower is promoted. The position their enclosure within the ovary of the carpel, and (in the of the sporophylls at the end of their axis gives them prominence, higher forms) within the torus itself, and to the pollen- and the whole arrangement of flower, whether solitary or m insacs by their investment in the anther; and (2) the florescences, at the extremity of shoots, and its projection, as provision of a carpellary germinating surface in the is common, beyond the vegetative organs is of obvious advantage. In contrast with the usual relation, cauliflorous trees are not stigma for the pollen-grains, with the correlated special- uncommon in the tropics. In them the flowers which have often ization of the stamen into filament and anther. The somewhat fleshy flower-envelopes, and are always odorous, appear flower may be constituted by the sporangiferous singly or in groups upon small short-lived twigs, which develop Flower- portion ai0ne, and the sporophylls are sometimes from persistent axillary buds, or are found issuing from latent envelopes. spoken of ag itg essential organs. But the parts eyes” in the bark. Their position close to the seat of reservefood is evidently an economical one, and may have a relation to which give conspicuousness to most flowers, and which are pollination, and their occurrence in tropical trees and absence indeed in the popular and gardening sense of the term from those of temperate regions may be made possible by the thinner bark developed upon trees in the tropics. The antagon“ flower ” the important ones, are the flower-envelopes observable between vegetative and reproductive functions is calyx of sepals and corolla of petals. One of these only may ism well known, and environmental conditions such as physiological be present. Their presence is a mark of angiospermous drought and low temperature, which are unfavourable to vegetative structure, the flower in Gymnosperms having only obscure activity, are not so unfavourable to, and even favour, reproduction. homologues. Sepals are typically green, and share therefore Periodicity in flowering is most marked where there is seasonal growth, and flowering commonly concludes a vegetative period in the assimilation-work of the plant, no doubt aiding in in which takes place an accumulation of energy sufficient to meeting the very great demands made upon the plant- supply the demands made in the seed-formation, the exhausting energy by spore-formation in the flower and its conse- nature of which is shown by the long intervals for recuperation, a dozen years, between successive full crops m such trees as quences. They are frequently persistent and active m this often beech. When flower is produced before seasonal yegetative activity way during the whole period of maturation of the seed. begins, a store of reserve-food from a preceding year is availWhilst in the earlier stages of the flower - development able for its formation. Where seasonal growth is not sharply they serve as protecting structures to the parts within, marked, as in parts of the tropics, there may be continuance ol flower during the whole year, but not from the same shoots. they also when persistent provide a protection, or it may The action of light in intensifying the colour and odour of alpine be an aid in dissemination, to the fruit or seed, ihe flowers is well known. _ _ u r +p brilliant colours of the flower are most frequently located The sporogenous tissue is formed within each ol the in the petals, although the sepals and sporophylls and four sporangia, two of which are in each lobe of the anther, adjacent parts are also often coloured. With the calyx the though there may be two only, or there may be , corolla serves as a protection to the young sporophy s, more. Each sporangium has the usual tapetum mcrospor ang}um, especially from wet. Once expanded the petals may and wall-layers, and these are eventually more remain so until they are shed, as they usually are after or less absorbed, as are also certain of the tissues ot e pollination has been effected, or they may with the calyx S. I. — 5 5
