44^ 



Fig. 396 A young jern plant still attached to 

 the prothallus. From what stnictiirc did it de- 

 velop? (ward's natural science establish- 

 ment) 



a prothalhis) which bears no resemblance 

 to the parent plant. This tiny plant lies 

 flat on the ground and has tiny hairs that 

 absorb water from the soil. Since it con- 

 tains chlorophyll and can make its own 

 food, it grows but never grows much 

 larger than a finger nnil. After a few 

 weeks, in certain groups of cells sperms 

 are produced; in other groups t^^j^ cells 

 are produced. The sperms swim through 

 the thin film of water on the lower sur- 

 face of the prothallus, and one sperm 

 fuses with each c<^^. The fertilized q^^ 

 cell then divides many times and differ- 

 entiation takes place. Each fertilized t^^ 

 becomes a small stem and root; and in 

 time the characteristic fern leaves 

 (fronds) rise up into the air. With their 



Hoiv Animals ami Plants Reproduce unit viii 



appearance reproduction is completed, 

 for new fern plants like the original fern 

 have been produced. 



It is interesting to note that before the 

 new fern appeared by sexual reproduc- 

 tion a little plant quite different from the 

 fern plant was produced by asexual 

 reproduction. There is always asexual 

 and sexual reproduction in regular rota- 

 tion before the fern cycle is complete. 

 This is called an alternation of genera- 

 tions: the fern plant produces spores 

 asexually; these grow into a prothallus; 

 the prothallus produces gametes; and the 

 fertilized egg grows into the new fern 

 plant. To see spores do Exercise ii. 



(Optional) Resemblance between re- 

 production in a flowering plant and a 

 fern. While a flowering plant seems to 

 be different from the fern in its repro- 

 duction, close study shows that it, too, 

 goes through an alternation of genera- 

 tions. The pollen grain develops from a 

 spore, a small spore (microspore). The 

 embryo sac develops from a large spore 

 {macrospore) . The pollen grain with its 

 tube and nuclei is a three-celled prothal- 

 lus, tiny and colorless, but a prothallus 

 just the same because it produces the 

 sperm cell. In the same way the devel- 

 oped embryo sac is a prothallus which 

 produces the egg cell. Here, as in the 

 fern, the gametes are found in prothalli 

 which grow from spores. These prothalli 

 form asexually. When the fertilization of 

 the egg takes place sexual reproduction 

 is occurring. In other words, fertilization 

 is tied up in regular rotation with an asex- 

 ual reproduction; there is an alternation 

 of generations in which both the sexual 

 and asexual stages take place right within 

 the blossom. So tiny are the spores and 



