DEVELOPMENT OF THE FLORAL ORGANS OF SWEET CLOVER.
Fig. 2.—Lengthwise sectional view of a very young flower of Melilotus alba, showing the relative development of the stamens and pistil. In the upper set of stamens the divisions of the mother cells are completed, while division is just beginning in the lower set of stamens. In the ovules the outer integuments are well started on their development, a, Anther; o, ovule; p, pistil. × 38.
The stamens of Melilotus alba and M. officinalis may be divided into two sets, according to their length and time of development. ([Fig. 2.]) The longer set extends about the length of the anthers above the shorter set, and the pollen mother cells in the longer set divide to form pollen grains at least two days earlier than those in the shorter set. At the time the pollen mother cells divide, the longer set of stamens is approximately three-eighths of a millimeter in length and the pistil about half a millimeter long. The stigma and a portion of the style project beyond the stamens, and this relative position is maintained to maturity. The pollen mother cells undergo the reduction division while the megaspore mother cells are just being differentiated and while the outer integuments are barely prominent at the base of the nucellus. The pollen grains are formed while the embryo sac is beginning to develop. The division of the megaspore mother cell does not occur until a number of days later, and the embryo sac is not mature until the flower is nearly ready to open. Thus, the pollen grains are formed a week to 10 days before the embryo sac is ready for fertilization. The pollen grains increase in size and undergo internal changes after their formation. These changes, which are not completed until the flower is one-half or more of its mature length, may be regarded as the ripening processes, and they are undoubtedly necessary before the pollen is capable of functioning. For this reason it is probable that the pollen grains are not able to function much before the embryo sac is mature.
Fig. 3.—Stigma at the time of pollination, showing its papillate character and the position of the pollen in reference to the papillæ in pollination. × 175.
The pistils of Melilotus alba and M. officinalis are straight for the greater part of their length, but curve rather abruptly toward the keel just below the capitate stigma. The surface of the stigma is papillate. ([Fig. 3.]) In their reaction with Sudan III, alkanin, and safranin the Walls of the papillæ of the stigma show that some fatlike substances are present. Aside from water, the contents of the papillæ consist chiefly of a fine emulsion of oil.
DEVELOPMENT OF THE OVULES.
The number of ovules in the ovary of Melilotus alba varies from two to five; however, most commonly, three or four ovules occur. In Melilotus officinalis the number in each ovary ranges from three to six. In both species the ovules are campylotropous at maturity with the micropylar end turned toward the base of the ovary.
Mature ovules contain two integuments, but the inner one does not close entirely around the end of the nucellus. The outer integument develops considerably ahead of the inner one. The outer integument is much thickened at the micropylar end, the seed coat is formed from it, and the inner integument is used as nourishment by the endosperm and embryo.