174 GENERAL BIOLOGY 



attached (as in the ordinary colony), while others 

 separated from the parent stock, and if the function 

 of reproduction should have become confined to these 

 latter colonies, we would have the condition just 

 described for the hydroid. Such a division of labor 

 between the parts of the colony (i.e. the individuals), 

 whereby one particularly specialized group of in- 

 dividuals reproduces sexually and the rest asexually, 

 is known as metagenesis or alternation of generations, 

 since the sexual individual resembles, not its im- 

 mediate parent nor its descendant, but, so to speak, 

 its grandparent or grandchildren. The asexual 

 stage may, however, include more than one individ- 

 ual, particularly in those types that reproduce by 

 fission or budding. Such an alternation of sexual 

 and asexual generations occurs in widely separated 

 groups of animals, probably having arisen independ- 

 ently in all of them. 



SEXUAL REPRODUCTION IN PLANTS 



In the simplest plants, as in the simplest animals, 

 vegetative or asexual reproduction is the rule, with or 

 without spore-formation. In the bacteria, and some 

 green algae, no other method is known. Of the lower 

 plants that live in water, the spores are frequently 

 provided with flagella and are motile. For this 

 reason they are called zoospores. The spores of 

 land-plants are, however, non-motile, although in 

 some groups they are so minute and light that they 

 float in the air and are borne everywhere by the wind. 



