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THE CONTINUITY OF THE RACE 



REPRODUCTION IN THE SPERMATOPHYTES 



In the seed plants (Spermatophyta), which constitute the fourth and 

 highest division of the plant kingdom, the alternation of generations has 

 become hard to detect. This is because the sporophyte generation has 

 become overwhelmingly preponderant, with the gametophyte generation 

 reduced to minute, short-lived structures nourished and borne by the 

 spermatophyte tissues. There are three main groups of seed plants— 

 cycads, conifers, and flowering plants. They differ in many details of 

 their reproduction, but the essential features of the gametophyte-sporo- 

 phyte relationship are the same in all and can be sufficiently illustrated 

 by a consideration of the flowering plants, or angiosperms. We shall have 



Fig. 17.8. A diagram to show the relative importance of the gametophyte and sporophyte 

 generations in the four major divisions of the plant kingdom. 



to begin by describing their characteristic reproductive structure, the 

 flower, in which the gametophyte develops and through which sporophyte 

 and gametophyte cooperate to perpetuate the race. 



The Flower 



The flower is a special reproductive adaption confined to and charac- 

 teristic of the highest group of seed plants, the Angiospermae. Essentially 

 it consists of an assemblage of parts derived from leaves, all more or less 

 modified, borne upon a modified twig, and concerned with the production 

 of the seed. 



The modified branch that bears the flowers arises from a bud situated 

 in the axil of a leaf, like an ordinary foliage branch. The leaf at the base 

 of the flower branch, however, is not usually of the same form as the 

 foliage leaves; it is called a bract. It is usually small and often deciduous 

 but sometimes enlarges to enclose the flower or become a part of the flower. 

 The flower-bearing axis may be either branched or unbranched. When 

 unbranched it bears a single flower at its apex and is called a peduncle. 

 Often, however, the axis is branched and the flowers are borne in clusters 

 or inflorescences. In this event the usually short stems of the individual 



