VOL. 1--I thallium or female gametophyte, consisting of a mass of rather uniform cells, and later pro ducing four or five eggs. The megaspore is the first cell of the female gametophyte. The pollen grain is carried by the wind to the female cone where it falls upon the sporangium and germinates, forming a pollen tube which grows down through the tissue of the sporan gium until it reaches the egg. Two sperms are formed within the pollen tube and one of these enters the egg and fertilizes it. As in the fern the fertilized egg is the first cell of the sporo phyte generation. The egg divides rapidly and becomes differentiated into root and stem re gions, while the tissues of the sporangium and surrounding structures become hardened into a seed coat. The seed then falls out from the cone and develops into the pine tree. In com paring this life history with that of the fern, it will be noted that in the fern there is only one kind of sporangium producing one kind of spore which gives rise to a gametophyte bearing eggs and sperms, while in the pine there are two kinds of sporangia, one producing spores which give rise to male gametophytes and the other producing spores which give rise to the female gametophytes. Plants with only one kind of spores are homosporous. Those with two kinds of spores are heterosporous. This con dition is derived from the homosporous and is found in all seed plants and in some ferns and lycopods.
In the sunflower the reduction of the gametophyte is still more extreme. The sun flower is composed of hundreds of very small flowers, each of which has five stamens and one ovary. The stamen bears four sporangia, each containing hundreds of microspores or pollen grains. The ovary contains one sporan gium within which one spore mother cell pro duces four megaspores, one of which germi nates, while the others abort. The prothallium formed by the germination of this spore is entirely included within the sporangium and is completely dependent. It is called the embryo sac and contains only eight nuclei, which at first lie free in the sac. Later, three of these in the narrower end of the sac become sep arated by walls. These three are called antip odal cells and, usually, they have no function. Two of the nuclei, called polar nuclei, fuse and form a large nucleus called the endosperm nucleus. The other three form a group at the
opposite end of the sac from the antipodals and constitute the egg apparatus. They become organized as more or less definite cells, but are surrounded only by a delicate pellicle. One of the three is the egg; the other two are called synergids, because they are supposed to assist in fertilization. The pollen grain is carried to the tip of the ovary by insects and there pro duces a long pollen tube containing two sperm nuclei. One of these nuclei fuses with the endosperm nucleus which then divides and pro= duces a tissue called the endosperm, constituting the principal bulk of corn, cereals and other seeds. The other sperm nucleus fuses with the egg, which, as in the other cases already con sidered, is the first cell of the sporophyte gen: eration. From the fertilized egg a new sun flower plant is produced. The gametophyte generation consists of two separate plants, the male and the female. The male, composed of the pollen grain and its tube, and the female, consisting of the embryo-sac, alternate with the sporophyte generation, the sunflower plant, which begins with the fertilized egg. Some features of the life history of the sunflower are shown in Fig. 4.
Below the ferns, in the mosses and liver worts, the gametophyte and sporophyte genera tions alternate just as regularly, the fertilized egg being the first cell of the sporophyte and the spore produced by the sporophyte being the first cell of the gametophyte; but the sporo phyte never becomes free from the gametophyte and, consequently, the existence of two genera tions is not so evident as in the ferns. The spore produces a branching filament called the protonema, upon which buds appear and de velop into the familiar leafy moss plants. Upon the leafy plant, often at the top, antheridia and archegonia appear, containing respectively the sperms and eggs. The whole structure from the spore up to this point constitutes the gametophyte. A sperm fertilizes an egg which then develops into a stalk bearing a sporangium, called a capsule, within which spore mother cells appear, each giving rise to four spores, the spore being the first cell of a new gameto phyte generation. The sporophyte begins with the fertilized egg and ends with the spore mother cell.