CHAPTER XXI 

 THE FLOWER 



A FLOWER is a shoot, or the termination of a shoot, 

 whose leaves (floral leaves) are specially modified in 

 different ways. The upper (inner) leaves produce the 

 reproductive cells, the lower (outer) ones serve to 

 protect the flower bud and (usually) to render the flower 

 conspicuous. The apical meristem of the flower bud 

 ceases its activity with the production of the floral 

 leaves, so that terminal growth in length of the floral 

 shoot is at an end. 



The seed plants are heterosporous, like Selaginella 

 (p. 251), and while one set of floral leaves bears micro- 

 sporangia containing microspores, another, set bears 

 megasporangia, each megasporangium normally con- 

 taining one megaspore. The prothallus produced by 

 each is greatly reduced and is formed together with the 

 gametes inside the spore. One of the essential characters 

 of seed plants is the means by which the gametes 

 are brought together. The microspores are carried by 

 external agency, usually insects or the wind, to a 

 special part of the floral leaf bearing the megasporangia. 

 There the microspores germinate, each putting out a 

 germ tube which, carrying the male gametes, grows 

 towards and penetrates the megasporangium, setting 

 free the male gametes inside the megaspore, in the 

 immediate neighbourhood of the female gamete or egg. 

 Conjugation thus takes place inside the megaspore. 



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