[Chap. XXXIII SEXUAL REPRODUCTION IN FLOWERING PLANTS 379 



the life cycle of the plant; and in the study of heredity it is generally 

 considered one generation. 



The life cycle of a flowering plant may be analyzed from another point 

 of view which may be stated briefly now, and then reconsidered after 

 the life cycle of a fern has been discussed. A plant in which micro- 

 spores and megaspores'^ develop may be termed a sporophyte (spore- 

 bearing plant). The parts of the flower in which spores develop are 

 often referred to as sporophylls (spore-bearing leaves); hence, the 

 stamens may be thought of as microsporophylls and the carpels of the 

 pistil as megasporophylls. 



Eggs and sperms, collectively called gametes,^ are also formed during 

 the life cycle of a seed plant. In ferns and mosses, gametes develop in 

 small multicellular plants that grow free upon the soil. Since these plants 

 bear gametes they are called gametophytes. If this terminology is applied 

 to seed plants, then the pollen grain plus the pollen tube in which the 

 sperms are formed is a male gametophyte (sperm-bearing plant), and 

 the embryo sac containing the egg is female gametophyte (egg-bearing 

 plant ) . 



The gametophytes of flowering plants are almost microscopic in size, 

 non-green, and parasitic upon the tissues of the sporophyte. The gameto- 

 phyte phase may be thought of as alternating with the sporophyte 

 phase in the life cycle. The life cycle then is composed of two phases, 

 each bearing the particular reproductive cells from which the other 

 develops. The endosperm, however, is neither gametophyte nor sporo- 

 phyte; it is termed the xeniophyte. 



Summary of the usual sequences of events in sexual reproduction. The 

 formation of fruits and seeds from flowers usually follows the sequence 

 of events described in the preceding pages. Summary statements of 

 these events are: (1) the formation of microsporocytes, microspores, 

 and pollen in regular sequence within the anther of the stamen; (2) the 

 development of two sperm nuclei from the generative nucleus within 

 the pollen grain or the pollen tube; (3) the formation of one or more 

 ovules in the ovulary of the pistil; (4) the formation in regular sequence 

 within each ovule of a megasporocyte, an active megaspore, and an 



* It is customary to use the term "microspore" to refer to the precursor of a pollen grain, 

 and the term "megaspore" to refer to the precursor of the embryo sac, even though the 

 terms are sometimes literally inappropriate, since in many flowering plants there is little 

 difference in size and sometimes the microspore is the larger of the two. 



* Eggs and sperms are called "megagametes" and "microgametes" respectively by some 

 authors. 



