Reproduction and Life Cycles in Higher Plants 51 



In the angiosperms, the sporophyte, commonly regarded as the 

 plant itself, bears two kinds of spores which in turn produce 

 two kinds of gametophytes. The male spores or microspores 

 are formed in the anthers of the flower. Cells towards the inside 

 of the anthers enlarge and become microspore mother cells or 

 microsporocytes. They undergo the usual two meiotic divisions, 

 and each forms four microspores. A microspore is a round cell 

 with one nucleus and, as it develops, it secretes about itself a 

 thick wall which is usually yellow and highly sculptured in such 

 a characteristic way that the species of plants can be identified 

 from the ridges and furrows of the microspore walls. A micro- 

 spore is a one-nucleate pollen grain; but soon after it is formed, 

 the nucleus and sometimes also the cytoplasm divide into a tube 

 nucleus or tube cell and a generative nucleus or cell. This two- 

 nucleate or two-celled pollen grain is a microgametophyte. 



The female spores form from megaspore mother cells or mega- 

 sporocytes, located in the ovules. The female organ is the pistil 

 of the flower. It is enlarged at the base into an ovary which 

 contains one or more ovules, each of which can develop into a 

 seed. Each ovule contains only one megasporocyte, and it divides, 

 by meiosis to form a row of four cells, each of which is a poten- 

 tial megaspore. Three of these cells degenerate and the fourth 

 enlarges to form a large functional megaspore or young embryo 

 sac. The nucleus divides by ordinary mitosis to form two, four, 

 and finally eight nuclei within the one embryo sac. Three nu- 

 clei collect at each end and two in the center, and cell walls are 

 formed about the three at each end. One of the cells at one end 

 of the embryo sac is the female gamete or egg. The embryo 

 sac at this stage is mature and is the female gametophyte or 

 megagametophyte. 



When the two-celled pollen grain is mature, it is liberated from 

 the anther and blows or is carried by insects to the end of the 

 pistil, where it adheres. The wall of the pollen grain bursts, 

 and the protoplasm grows out as a pollen tube which grows down 

 through the tissues of the pistil until it enters the ovary. The 

 pollen tube is a later stage of the microgametophyte. The tube 

 nucleus precedes and the generative nucleus follows farther be- 

 hind in the tube (Fig. 15). As the tube approaches the ovule^ 

 the generative nucleus divides by mitosis to form two sperm 

 nuclei or male gametes. The tube then passes through the micro- 



