338 



REPRODUCTION IN PLANTS 



that a comparable development goes on there. A con- 

 siderable number of spore mother cells develop in the cavi- 

 ties of each anther. Each single one undergoes the procr 

 esses of reductional division and produces four spores, just 

 as each sperm mother cell in the animal produces four 

 sperms. The spores which develop in the anther also be- 

 gin to grow and usually contain two or three cells at the 

 time they are shed and carried by wind or insect to the tip 

 of the pistil. Here they produce elongated protrusions of 

 their walls called pollen tubes, which bore their way down 

 through the tissue to the cavity of the pistil where the 



Fio. 01. — The Pollen. This figure Hhows a cross section through 

 the anther at the top of a stamen. There are four cavities in which 

 thousands of pollen grains are formed. Each pollen grain is a micro- 

 spore when first formed but usually has germinated to form a two- 

 celled male gamctophytc Ixifore it is actually shed and carried to 

 the tip of the pLstil. (See Fig. 92) 



ovules are, and enter a minute opening in the ovule, 

 known as the micropyle, and eventually reach the egg 

 inside. In the meantime the pollen grain has completed 

 its growth into a microscopic male gametophyte. It is so 

 reduced in fact that it normally consists of only three cells. 

 One of these is the so-called tube nucleus which appears 

 to direct the action of the pollen tube and the other two 

 are sperms. 



Fertilization, Embryo, and Endosperm (Fig. 93). — 

 When the two sperms are discharged from the pollen tube 



