328 



ELEMENTARY BIOLOGY 



of those species that protect their young ; but it is not probable 

 that in these species so large a proportion are lost. 



380. Water essential to gametes. As we have seen, sexual 

 reproduction is possible only on condition that two gametes of 

 opposite sex combine. In addition to producing the gametes, 

 the bringing of them together is another problem of life. 



Moreover, the sperm and 

 egg cells (gametes) are 

 unlike spore cells in that 

 they are quite incapable of 

 resisting drought ; drying 

 kills them very quickly. 

 It is therefore another 

 condition of reproduction 

 that the gametes be pro- 

 tected against drying up. 

 Among the animals and 

 plants that live in the 

 water, or where water may 

 remain in contact with 

 their reproductive organs, 

 this is simple enough. 

 But in organisms that 

 live on land, or in the 

 air, the older methods 

 of bringing the gametes 

 together will no longer serve. We have seen how this con- 

 dition is met in the case of the flowering plants. Among 

 land animals there are special organs and modes of behavior 

 that make fertilization possible. 



381. Reproduction among batrachians. The frogs, which 

 live on land and breathe air in their adult state, go to the 

 edges of ponds and puddles at the breeding season. After the 

 gametes are thrown into the water, fertilization takes place, and 

 the adult frogs pay no further attention to them. 



Fig. 155. Hydromedusa {Boiigainvillea 

 7'ai)iosa) 



a, enlarged view of portion of colony, showing 

 feeding individuals and reproduction individuals. 

 New individuals are here produced asexually, by 

 budding. /', the medusa stage, which originates as 

 a bud on the hydra colony and reproduces by means 

 of gametes thrown into the water 



