CHAPTER XXXIII 

 SEXUAL REPRODUCTION IN FLOWERING PLANTS 



Sexual reproduction occurs in nearly all kinds of plants. In angiosperms, 

 commonly called flowering plants, and in gymnosperms (pines, firs, 

 spruces, and other cone-bearing plants ) it is associated with the forma- 

 tion of seeds. With a few exceptions the processes of sexual reproduction 

 and seed formation in these two groups of plants are similar, but in this 

 chapter all statements about seeds and seed plants refer particularly to 

 angiosperms. 



A complete account of the principal events in the sexual reproduction 

 of seed plants must include a description of how the embryo and endo- 

 sperm come to be in the seed, how pollen develops in the anther, and 

 how pollen is related to the formation of seeds and fruits. 



Since seeds develop from ovules, and since the seed coats, embryo, 

 and endosperm are each composed of cells, one may infer at once that 

 each part of the seed grows from certain cells in the ovule. The origin of 

 the embryo of a seed plant, like the origin of the embryo of other organ- 

 isms, usually is a fertilized egg — a cell formed by the union of an egg 

 and a sperm. 



From what cells, then, do the eggs and sperms develop in a seed 

 plant? What is the precursor, or forerunner, of the endosperm of a 

 seed? Of the pollen grain in the anther? Each of these several struc- 

 tures develops from others in a definite order or sequence; and to know 

 the story of sexual reproduction one must learn which processes and 

 structures are the precursors of others. 



Deviations from the usual sequence may occur. The results of some 

 of these deviations are famihar objects. Seedless fruits, seeds without 

 endosperms, seeds with more than one embryo, identical twins, and 

 abscission of pistils are consequences of certain departures at some par- 

 ticular stage in the usual story of reproduction. One must be able to 

 visualize this usual story in order to appreciate such deviations as those 

 just mentioned, to compare sexual reproduction in seed plants with that 

 of other organisms, and to understand its relation to heredity. To do this 



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