SPERMATOPHYTES : ANGIOSPERMS 



201 



any organization of carpels which ap- 

 pears as a single organ with one ovary 

 is a pistil. 



The ovules (megasporangia) are 

 developed within the ovary (Fig. 170) 

 either from the carpel wall, when they 

 are foliar, or from the stem axis which 

 ends within the ovary, when they are 

 cauline (see § 89). They are similar 

 in structure to those of Gymnosperms, 

 with integument and micropyle, nu- 

 cellus, and embryo -sac (megaspore), 

 except that there are often two integu- 

 ments, an outer and an inner (Fig. 

 171). 



112. The male gametophyte. — When the pollen-grain 

 (microspore) germinates there is form ed within it the sim- 

 plest known g ametophy te (Fig. 172). Xo trace of the 



Fig. 171. A diagrammatic 

 section of an ovule of 

 Angiosperms, showing 

 outer integument (ai), 

 inner integument (it), 

 micropyle (m), nucellns 

 (k), and embryo sac or 

 megaspore (em). — After 

 Saces. 



Fig. 172. Germination of microspore (pollen grain) in duckweed (Lemna): A, mature 

 spore with its nucleus; B. nucleus of spore dividing; C, two nuclei resulting from 

 the division; Z>, a large and small cell following the nuclear division, forming the 

 two-celled male gametophyte; E, division of smaller cell (generative) to form the 

 two male cells; F, the two male cells completed and lying near the large tube 

 nucleus.— Caxdwkli.. 



