FUNCTIONS OF LIVING THINGS 



53 



Without the formation of new living things no progress would be 

 possible on the earth. We have found that insects help flowering 

 plants in this process. Let us now see exactly what happens 

 when pollen is placed by the bee on the stigma of another flower 

 of the same kind. To understand this process of reproduction in 

 flowers, we must first study carefully pollen grains from the anther 

 of some growing flower. 



Pollen. — Pollen grains of various flowers, when seen under the 

 microscope, differ greatly in form and appearance. Some are rela- 

 tively large, some small, some rough, others smooth, some spherical, 



Pollen grains of different shapes and sizes. 



and others angular. They all agree, however, in having a thick 

 wall, with a thin membrane under it, the whole inclosing a mass 

 of protoplasm. At an early stage the pollen grain contains but a 

 single cell. A little later, however, two nuclei may be found in the 

 protoplasm. Hence we know that at least two cells exist there, one 

 of which is called the sperm cell ; its nucleus is the sperm nucleus. 

 Growth of Pollen Grains. — Under certain conditions a i)ollen 

 grain will grow or germinate. This 

 growth can be artificially produced in 

 the laboratory by sprinkling pollen 

 from well-opened flowers of sweet pea 

 or nasturtium on a solution of L5 

 parts of sugar to 100 of water. Left 

 for a few hours in a warm and moist 

 place and then examined under the 

 microscope, the grains of pollen will 

 be found to have germinated, a long, 

 threadlike mass of protoplasm grow- 

 ing from it into the sugar solution. 



A pollen grain greatly magnified. 

 Two nuclei are found {n, n) at 

 this stage of its growth. 



