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A TEXT-BOOK OF BOTANY. 



The Megasporophylls consist of sessile carpels (leaves) on 

 which are borne one or two naked ovules containing the sporangia 

 (nuclei). In certain groups, as in the pines, balsams, etc., a 

 scale is formed at the base of the carpel which bears the ovules, 

 and this scale is called the seminiferous scale. The ovules con- 

 sist of several parts (Figs. 70 and 71) : a stalk ; an integument or 

 wall which has an opening at the apex known as the micropyle ; 

 a nucellus (megasporangium), being that portion next within the 



FIG. 69. A, longitudinal section of cone composed of microsporophylls, of one of the 

 pines; B, longitudinal section of microsporophyll showing microsporangium (pollen sac); 

 C, the same in transverse section showing both microsporangia; D, winged microspore 

 (pollen grain), with a two-celled male gametophyte, the upper cell being the generative 

 cell, the remaining nucleated cell giving rise to the pollen tube. After Schimper. 



integument ; and embedded within the nucellus a portion known 

 as the megaspore or embryo-sac. 



Gametophytes. The development of the gametophytes from 

 the asexual spores, namely, the microspore or pollen grain, and 

 the megaspore or embryo-sac, is as follows : The nucleus of the 

 megaspore divides repeatedly (Fig. 71), cell walls are formed, 

 and a multicellular structure known as the ENDOSPERM is pro- 

 duced. This structure constitutes the prothallus of the female 



