GYMNOSPEEMS 



333 



development shows that each megaspore is produced by a single 

 mother cell, which lies deeply buried in the tissue of the young 

 sporangium. This mother cell divides by tetrad division, and one 

 of the cells of the tetrad forms the single successful megaspore. 

 The ovules of the spruce, like those of the cycad, are therefore 

 true megasporangia, in which a single megaspore, produced by 

 the usual processes of spore formation, is formed and permanently 



tegasporophylli 



Microsporophyll 



Megasporangi 



Strobilus 



Microspores 



FIG. 197. Megasporangia and microsporangia with spores 



a, megasporophyll (adaxial view) with two ovules (megasporangia); 6, mega- 

 sporangium in median section, showing the single megaspore ; c, portion of a stro- 

 bilus in section, showing megasporophylls, bracts, megasporangia, and spores; 

 d, male strobilus ; e, microsporophylls and sporangia ; /, microsporophylls, spores, 

 and sporangia (sectional view) 



retained within the sporangium, instead of being shed, as in Selag- 

 inella. As the megaspore enlarges, .it germinates and produces a 

 true cellular gametophyte (Fig. 198, a). After fertilization the 

 megasporangium becomes the seed, furnished with a hard seed 

 coat, or integument, and an embryo spore plant produced by the 

 fertilized egg. 



GAMETOPHYTES AND EMBRYO 



The male gametophyte in the spruce is similar to that of the 

 cycads and is formed within the microspore (Fig. 198, d) as 

 a result of germination. It consists at first of two cells (the 



