VEGETATIVE PROPAGATION AND REPRODUCTION 615 



duction, since the new organism is formed by a cell or cells separat- 

 ing from a single parent. Other examples of this type of repro- 

 duction is seen in the reproduction of yeast or mold, which form 

 asexual spores. Such spores under favor- 

 able conditions produce new plants. 



New living organisms, however, are usu- 

 ally formed by other methods. If two cells, 

 from two plants or animals of different 

 sexes, come together to form a new indi- 

 vidual, we call this a case of sexual repro- 

 duction. In some plants, like Spirogyra, 

 already studied, a kind of sexual spore is 

 formed by a process we call conjugation. 



Two filaments, lying side by side, send 

 out little projections from adjoining cells, 

 Which meet, and the contents of the cells 

 in one filament pass over and mix with the 

 cell contents of the other filament. The 

 cells thus formed become resting spores 

 (zygospores) which will develop into new 

 plants. The cells which formed these 

 spores are called gametes. 



In the algae we have a step higher in 

 the development of gametes. In the Vau- 

 cheria (vo-ke'ri-d), a branched alga, two 

 structures may be produced from the fila- 

 ment. One contains a large gamete which 

 is called an egg. The other contains small 

 gametes, called sperms, each with two cilia. 

 The sperms when set free appear to be 

 chemically attracted to the egg cells. If a sperm (male gamete) 

 fuses with an egg (female gamete), it is said to fertilize the egg cell. 

 From the fertilized egg a new plant will eventually grow. This 

 method of development, which is found in all higher plants and 

 animals, is known as sexual reproduction. 



We have already learned how sexual reproduction takes place 

 in the flowering plants. The flower holds pollen grains and ovules 



Conjugation in Spirogyra. 

 Explain the diagram. 



