Page:The American Cyclopædia (1879) Volume XIII.djvu/604

 588 PLANT sac, which is filled with mucilaginous material. If the ovule is not fertilized, i. e., does not receive the influence of the pollen, it with the ovary develops no further. If pollen falls upon the stigma the pollen tubes are pro- truded and make their way down through the loose tissue of the style, probably receiving nutriment from that to promote their growth, until the embryo is reached, where the act of fertilization takes place. Physiologists are not agreed as to precisely in what the act of fertiliza- tion consists ; the pollen tube has been traced from the stigma down through the tissues of the style to the foramen of the ovule ; after this takes place there appears within the nucleus of the ovule an embryonic vesicle, constituting the commencement of a new plant, which be- gins with a single cell, formed through the in- fluence of the pollen, and this by its increase through subdivision develops into an embryo or young plant, the coats of the ovule at the same time enlarging and changing to become the coats of the seed. But the changes in the embryo are not the only ones which follow fertilization ; the ovary itself greatly enlarges and alters, and becomes the fruit, a name in botany applied to the ripened pistil and the parts attached to it, without reference to edi- ble or other qualities. With the completion of the embryo the seed and fruit are mature. Morphology of the Flower. Before describing the fruit, reference must be made to other modifications in the flower as well as to the nature of the flower itself. The adhesion of parts of the same series has been described, but there is another union of parts which not only greatly modifies the flower and obscures its structure, but which often has a marked influence upon the character of the fruit. This is the cohesion of parts of different series: the sepals and petals may not only unite among themselves, but the calyx may unite with the corolla for a portion of their length ; this union sometimes includes the itamens (e. g., peach and cherry), in which case the petals and sta- mens appear to be inserted upon the margin of the calyx tube, though structurally all are regarded as having their origin on the recep- tacle, and cohering for a part of their length. This union may proceed still further, and in part or in whole involve the ovary (e. g., apple and pear), when the other parts of the flower appear as if inserted upon the top of the ovary, giving rise to the term inferior ovary of the older botanists. It sometimes occurs that the calyx and corolla remain distinct, while the sta- mens and pistils are blended in one mass (see ORCHIDS) ; such flowers are said to be gynan- drous. In describing the organs of vegetation some of the -many forms assumed by root, stem, and leaf have been pointed out, and it has been shown that these are often turned aside from their ordinary uses and made to serve some entirely different one. Now when the plant is ready to provide for its reproduction by seed, is an entirely new set of organs called into existence, or are those with which the plant is already provided made to serve for this pur- pose also ? What has been shown of the mor- phology of the organs of vegetation has pre- pared the way for the statement that a flower is only a peculiarly developed branch, and that its parts are only modified leaves. The theory of metamorphosis, as it is termed, does not as- sume that the parts of the flower were formed first as leaves and then transformed into sepals, petals, &c., but that they are homologous with leaves ; and that plasticity which allows a leaf to appear at one time as a tendril, at another as a spine, again as a pitcher, or as a most effi- cient trap for catching insects, will allow it, when the plant requires such organs as stamens and pistils to produce seed, and calyx and co- rolla to aid in the work, to assume these forms, which in many cases are not further removed from the normal condition than occurs when there is no doubt that the organ is still a leaf. But a few of the many illustrations in support of this theory can be cited. It has been shown that flower buds, and consequently flowers, ap- pear in the same place as do leaf buds termi- nating the principal stem, or sessile, or on small branches in the axils of its leaves ; also that when a flower cluster is formed it results from a modification of the usual manner of branch- ing. The flower terminates the stem upon which it is placed, whether the main stem or flower stalk, the usually enlarged and rounded end of the stem becoming the receptacle, upon which the parts of the flower are placed, one above another ; in viewing a flower as a modi- fied branch, we find its internodes very short, but scarcely shorter than in the houseleek, where they are so abbreviated as to bring the leaves so close together that they touch ; indeed, except in color, some plants of that family closely resemble the flower of a double camel- lia or rose. Abnormal developments often throw much light upon the real nature of the flower and its parts, in which, the vital forces of the plant being from some cause disturbed, there is a reversion of the parts to the leaf or toward it, or retrograde metamorphosis as it is termed. Cases have been already cited in which the receptacle is prolonged in the centre of the flower to make room for the numerous pistils ; but it not rarely happens, in the rose, for example, that the receptacle is abnormally prolonged and assumes the true character of a branch, growing several inches in length, bear- ing true leaves, and being terminated by an an- other flower bud. The writer had a specimen in which this was repeated a third time on the same flower. The calyx is so generally green that its leafy nature is manifest, and often in the rose the divisions of the calyx become pin- nate, like the ordinary leaves. The resem- blance to green leaves is less manifest in the petals, as their more delicate texture and often brilliant colors aid in disguising them; but these colors and texture are often quite as striking in bracts, which are unmistakably