302 



ESSENTIALS OF BOTANY 



evident without careful examination. The pistil is the 

 megaspore-producing leaf, or megasporophyll, and the sta- 

 men is the microspore-producing leaf, ot miorosporojjhyll. 

 Pines and other gymnosperms produce a large cell (the em- 

 bryo sac) in the ovule (Fig. 218) 

 which is the megaspore, and a 

 pollen grain which is the micro- 

 spore. In its development the 

 megaspore produces an endo- 

 sperm or small cellular prothal- 

 lium, concealed in the ovule. The 

 microspore contains vestiges of a 

 minute prothallium. 



In the angiosperms the mega- 

 spore and its prothallium are still 

 less developed, and the micro- 

 spore, or pollen grain, has lost 

 all traces of a prothallium and 

 is merely an antheridium which 

 sooner or later produces two 

 generative cells. These are most 

 easily seen in the pollen grain, 

 but sometimes they are plainly 

 visible in the pollen tube (Fig. 

 123, B). 



Seed-plants are distinguished 

 from all other plants by their 

 power of producing seeds, or enclosed megai<porangia with 

 embryos. 



384. The Sexual Generation and Relationships of the 

 Great Groups of Plants. — On summing up Sects. 381- 

 S83 it is evident that the sexual generation in general 



i"iG. 218. Longitudinal Section 

 tiirough Fertilized Ovule of a 

 Spruce. 



p, pollen grains ; t, pollen tubes ; 

 n, neck of the arcbegoniuni ; 

 II, body of archegonium with 

 nucleus ; «, embryo sac filled 

 with endosperm. 



