442 



normally in relatively few plants. In 

 some of these the flower does not open 

 until after the pollen has been shed. Most 

 natural pollination is likely to be cross- 

 pollination by wind or insects. 



The pollen grain. Pollen is easy to 

 study under the microscope. See Exer- 

 cise 4. Most pollen grains, when ripe, 

 have two nuclei, so really there are two 

 cells within the thick wall. These cells 

 are not separated by a membrane, how- 

 ever. If a pollen grain happens to fall on 

 the top of a pistil in a flower of the same 

 species, a tube grows out of the grain as 

 shown in Figure 391. This is the pollen 

 tube; it grows down through the style 

 and enters the ovary. It can do this be- 

 cause the tip sends out digestive enzymes 

 that digest the tissues of the style and 

 ovary just in front of the growing tube. 

 As the tube grows, the two nuclei move 

 into it. In the meantime, one of the two 

 nuclei divides again, forming two sperm 

 nuclei. Thus there are three nuclei. One 

 of the sperm nuclei with its surrounding 

 bit of cytoplasm is the male gamete, cor- 

 responding to the sperm cell of the ani- 

 mal. Its twin sperm nucleus may eventu- 

 ally unite with the double nucleus. The 

 third nucleus, known as the tube nucleus. 

 remains at the tip of the tube. We need 

 be concerned only with the male gamete 

 (the sperm) which takes part in sexual 

 reproduction. You can see pollen tubes 

 with their nuclei by doing F.xercise 5. 



We have found the two ijametes: the 

 sperm hidden within the pollen tube and 

 the egg cell within the ovule. How do 

 these two cells get together in fertiliza- 

 tion? 



Fertilization. You have read that by 

 pollination pollen is brought from the 



Hoii: Ann) ids and Plants Reproduce unit viii 



Sperm 

 nucleus 



Sperms 



Tube nucleus 



Fig. ^91 A polleji grain iinich magnified. How 

 many nuclei are there in B.'' What is happening 

 in C? In D? How many nuclei are there in D.? 

 Which o7ies of these are gametes? 



stamen to the stigma of the pistil. The 

 pollen grain absorbs some of the sugary 

 fluid secreted by the cells of the stigma 

 and grows rapidly, forming a tube ^\•ith 

 three nuclei. This tube grows longer. It 

 secretes enzymes which dissolve the cells 

 lying in its path as it grows down through 

 the stigma and the style. It enters the 

 ovarv with its ovules. 



The growth of the pollen tube may 

 take from several hours to many days; 

 the time varies from plant to plant. Hun- 

 dreds of pollen grains may fall on the 

 stigma. Exercise 6 will show you that 

 many of these begin to form tubes. 



Eventually, one or more of the tubes 

 reaches the ovules in the ovary. Each 

 ovule has an opening (the iiiicropyle). 

 When a tube reaches the opening it en- 

 ters the ovule. The pollen tubes which 



