The sperm nuclei are carried \o the egg nucleus within the pollen tube of 

 the male gametophyte as it grows into the pistil. 



Fertilization occurs when the two sexual nuclei combine. 



After fertilization the mass of the embryo sac absorbs food and grows into 

 an embryo; the surrounding walls of the ovule become the seed coats. 



Fertilization also brings about the ripening of the ovary into a fruit; in 

 some plants the calyx and even the receptacle become fused into the fleshy 

 fruit. 



In some species the flowers are usually or always self-pollenated; in others 

 they are cross-pollenated. 



Many flowering plants depend upon external agencies, such as wind or 

 flying insects, to bring about pollenation. 



The coloration, specialized structures and odors of flowers may be consid- 

 ered as secondary sex characteristics, since they are but remotely connected 

 with the formation of the gametes. 



Thousands of species would not survive the winter but for the hard, inert 

 seeds through which they renew themselves when conditions again make 

 growth possible. 



Seeds are scattered in a variety of ways. 



The offspring of flowering plants have the advantages of a good food supply 

 and a wide dispersal in the well-protected seeds produced by the parent. 



EXPLORATIONS AND PROJECTS 



1 To find out how reproduction takes place in flowers, examine some com- 

 plete, regular, perfect flower, such as a wild rose, sedum, tulip, evening primrose, 

 geranium, forsythia, apple, lily, or gladiolus. Identify the stamens and the pistil, 

 and compare these with stamens and pistils of other species. Open the ovary to 

 locate the ovules; note their attachments and their arrangement in the one or 

 several carpels. Identify the outer accessory parts, sepals and petals, the parts 

 respectively of the calyx and the corolla. Identify the various structures in as many 

 different species as time permits. 



2 To find out how pollen carries the sperm to the ovule, germinate pollen 

 grains and examine under the microscope.^ Note the tubes projecting from some 

 of the grains. Look for distinguishable structures — the sperm nuclei — within the 

 protoplasm. Apply a little iodine or other stain to the side of the cover slip, to 

 make the sperm nuclei more easily visible. Make longitudinal sections of some 

 pistils to locate pollen tubes within. Relate the growth of the pollen tube to bring- 

 ing the sperm from the stigma to the ovule within the ovary. 



^To germinate pollen grains, rub them from stamens into a drop of a 3 per cent sugar 

 solution on a microscope slide. Cover with a cover glass and set aside at room temperature 

 in a moist chamber or germinating dish for twenty-four hours. 



414 



