420 



STRUCTURE AND LIFE HISTORIES 



that the ovuliferous scale represents two sporophylls 

 fused together. 



372. Megasporangia. As in Cycas, the megaspor- 

 angium is an ovule, comprising a nucellus (sporangium 

 proper), surrounded by an integument. At the apex the 

 integument does not come quite together, so that a 



' O.S. 



FIG. 310.- 



-Median longitudinal section of a pine cone, o.s, ovuliferous 

 scale; ov, ovule at the base of a scale. 



tiny opening or pore (the micropyle) is left. The micropyle 

 leads to the pollen-chamber below (Figs. 309 and 311). 

 The megaspores are more backward in development than 

 the microspores, the spore-mother-cell not appearing until 

 June. Reduction by two divisions gives rise to either 

 three or four megaspores in a row, but only the basal one 

 ever germinates; the others become disorganized and 

 furnish nourishment for the one, that germinates. 



