26 



INBREEDING AND OUTBREEDING 



highest and most specialized group, the seed plants, while 

 in the highest group of animals, the mammals, only an 

 occasional individual showing rudimentary hermaphro- 

 ditism is found. 



Just when sexual reproduction first originated in the 

 vegetable kingdom is even more of a question than among 

 animals. Only a few very simple types, the schizophytes 

 (bacteria) and myxomycetes, have passed it by. Perhaps 



FIQ. 4. 



Fio. 4. Rhopalura, an example of extreme sexual dimorphism. (After Caullery.) 

 FIG. 5. Sexual reproduction in Fwcus, giving some idea of the difference in size of 



egg and sperms. Sperms should be about one-tenth the size shown. (Bergen and Davis 



after Thuret.) 



it is for this reason these forms have remained the sub- 

 merged tenth of the plant world. It is tempting, as 

 Coulter 32 says, to see the origin in the Green Alga. 

 There, in certain species, of which Ulothrix is an example 

 (Fig. 6), spores of different sizes are produced. The 

 large ones having four cilia are formed in pairs in each 

 mother cell, the smaller ones usually having two cilia 

 occur in groups of eight or sixteen in each spore-produc- 

 ing cell. Those largest in size germinate immediately 

 under favorable conditions and produce new individuals. 



