372 TEXTBOOK OF BOTANY 



we need to know only what happens in the stamen and pistil of the 

 flower. Sepals and petals arc merely accessory structures. We may begin 

 with the stamen. 



What takes place in the stamen? At certain seasons of the year the 

 stamens become mature, their anthers open or dehisce, and from them 

 millions of powderv grains known as pollen or pollen grains are released. 

 What is their origin and how have they developed? To answer this 

 question it will be necessary to begin with a young anther. 



In a cross section of a verv young anther the cells look alike; but in 

 sections of slightly older anthers there may be seen four groups of 

 meristematic cells quite distinct from the remaining cells, and known as 

 the sporogenoiis (spore-bearing) tissue (Fig. 165). Further differentia- 

 tion in this tissue results in special spore-producing cells called micro- 

 spore mother cells, or microsporocytes. Each mother cell becomes sepa- 

 rated from the others and divides. The anther now contains numerous 

 cells attached in groups of two (dyads). When these cells divide, the 

 resulting daughter cells appear in groups of four (tetrads) and are 

 known as microspores. Each microspore mother cell, therefore, is really 

 the grandmother cell of four microspores. At maturity the microspores 

 fall apart and are seen as separate cells. Many microspores are ordinarily 

 formed in an anther at about the same time, and the parts of the anther 

 enclosing them are microsporangia (microspore cases). 



Each microspore at first contains a single nucleus. When this nucleus 

 divides, the microspore technicallv becomes a 2-nucleate pollen grain 

 and the microsporangium may now be termed a pollen sac. One of the 

 two nuclei in the pollen is the forerunner of the sperms and is known 

 as the generative nucleus; the other one is referred to as the tube 

 nucleus because of its relation to the growth of the pollen tube from 

 the pollen grain. A subsequent division of the generative nucleus,^ either 

 in the pollen grain or in the pollen tube, results in two male gametes, or 

 sperms. Dehiscence of the anther and shedding of the pollen occur in 

 most plants before the sperms are fomied. In the anthers of many 

 plants the wall between certain pollen sacs may open so that b}' the time 

 of dehiscence of the anther the original four microsporangia have be- 

 come two pollen sacs. 



We shall interrupt the story of what is happening in the stamen 



^ These nuclei are surrounded by cytoplasm, but the individual cells are not always 

 enclosed by distinct cell walls. It is more convenient, therefore, to observe and refer to the 

 nuclei than to the cells of which they are a part. 



