The petals and sepals may be entirely free from each other in the same whorl or their bases may be united, leaving the remainder free, as with the corolla in Old-Man's-Beard (Chien anthus), or the union may be practically com plete, as in the corolla of Morningglory (Ipomcra). Various other unions may be found in parts of flowers. Thus in the pea family (Fabacea.) very commonly the filaments of nine of the stamens are united into a tube which is open at one side, the tenth stamen standing in this opening but not united to the others. In the Composites (Asteraccce, etc.), the anthers of the five stamens are united into a ring around the stigmas, the filaments remaining free. In a great many gamopetalous flowers and some others the filaments are grown fast to the inner face of the corolla so that the stamens seem to have their origin from this instead of from the torus. In the Orchids (Orchidacem) the two (or one) stamens are united with the style to form the peculiar structure called the column.
The simplification of the flower by the re duction in the number of parts may be very great. In some species of maples the flowers have showy petals and large sepals; in other species the petals are lacking and the sepals rather small; in still others the sepals are mi nute. The elm flower lacks petals but has rudi mentary sepals; the flowers of the spurge (Euphorbia) lack both petals and sepals. In deed the latter are the simplest flowers known, consisting respectively of a single stamen or a single pistil for the male or female flower. That this is not the simplicity of primitiveness is shown by the fact that the pistil is compound and that the near relatives within the same family show all degrees of reduction from flowers with all parts, those without petals and those with sepals missing also. Along with such reduction of parts often goes the segre gation of sexes into different flowers on the same plant) or upon differ ent plants (diavions plants).
As flowers are reduced to greater and greater simplicity we find that there is a tendency to ward being more and more crowded together, e.g., catkins of willow and birch, or very much more numerous, e.g., grasses, elm, maple, etc. This reduction in complexity of structure and increase in numbers often goes hand in hand with the loss of insect pollination (entomophily) and the acquirement of anemophily (pollination by air borne pollen).
Flowers may be single on long or short stalks or in various types of clusters. (See INFLORES cmsE). Frequently they are associated with modified leaves, called bracts. These are some times brilliantly colored, adding to the conspicu ousness of the cluster. Sometimes the flowers themselves are inconspicuous and only the bracts arc showy. Such is the case with the cultivated Poinsettia, Snow-on-the-Mountain, etc.
Pollination.—Especial interest attaches to those modifications of floral structure which provide for pollination or prevent self-pollina tion while favoring cross-pollination. Those modifications that provide for pollination by means of insects are discussed more fully in the article FLOWERS still hisscrs.
It was shown by Darwin that some flowers are infertile or only partially fertile when pol lenized with their own pollen; on the other hand plants like wheat, bean, pea, etc., are reg ularly fertilized within the bud before the flower opens, by their own pollen without any discoverable loss of fertility or vitality. Many
flowers have contrivances which prevent self pollination. Thus the stigma may he receptive at a different time from that when the pollen is being shed. In this regard the flower may he proterogynous, with the pistil receptive be fore the pollen is set free, or proterandrous, in which case the pollen has all been shed before the stigma is receptive; the plantain being an example of the former, a comparatively rare condition, the proterandrous flowers being very ,numerous.
Certain plants, such as the violet. iewel weed (Impatiens), etc., have two kinds of flowers, the showy ones which are visited by insects and cross pollinated, and which may or may not produce seeds, and the so-called cleistogamous flowers, which remain small, are rudimentary in structure (petals often lacking), and do not open, pollination taking place within the unopened flower by the setting free of the pollen from the anthers so that it comes into direct contact with the stigmas. In some plants it is only the cleistogamous flowers that produce seeds. For a number of cases it has been demonstrated that the production of one or the other kind of flower depends upon the nutrition of the plant. • Evolution of the Flower.— The old idea that the flower represents a modification of a leafy shoot, with the sepals, petals, stamens and pistils as successively further and further de viations from the leaf structure has been aban doned since it is clear that the reproductive leaf (sporophyll) is probably older in the evolution ary history of the vegetable kingdom than the strictly vegetative leaf. In most ferns the leaves serve a dual purpose, that of reproduc tion (by producing spores) and of food manufacture. As we advance along the line of evolutionary development we find that there is a progressive sterilization of part of these leaves until the re productive function is confined to only a few leaves (sporophylls), the remainder of the having lost the capacity to produce spores and being the vegetative leaves. These sporoph).lis may resemble the vegetative leaves in being able to manufacture food, and in that case usually differ from them very little in appear ance. However, once the cleavage has ap peared we find that it progresses until they are very different, even in some groups not very far advanced. Along with this differentiation there occurred a segregation of the sporophylls from the vegetative leaves; thus in the ostrich fern the vegetative leaves arc produced earlier in the season and the sporphylls in a cluster in the centre toward the season's close. In some of the extinct forms that are on thepath to ward the higher plants (Bennettitales) the sporophylls are of two kinds, producing respec tively megaphores (female spores and micro spores (male spores) on the megasporophylls which are crowded together at the apex of a modified shoot, below which the micro sporophylls occur, also in large numbers, the whole being subtended by several large pro tective leaves. This type of flower is so sug gestive of the flower of Magnolia that many botanists now see in the latter the most primitive type of the modern Anthophyta. A further dis cussion of the lines along which evolution carried the modification of this type of flower structure will be found in the article PLANTS, MORPFIOLOCICAL EVOLUTION OF.