Factors in Evolution 45 



move towards the source of oxygen, and under a 

 cover glass may be seen to assemble near the 

 edge of the glass where the oxygen is more abun- 

 dant. They also will quickly gather about green 

 plant cells where oxygen is being given off under 

 the influence of light. Bacteria are also power- 

 fully attracted by certain organic solutions. Chemo- 

 taxis plays an important part also in the attraction 

 of the motile male cells, or sperms, to the eggs. It 

 has been shown by Pfeffer, in the case of some of 

 the ferns, that when the archegonium, the organ 

 containing the egg cell, opens, a substance is ejected 

 which contains some combination of malic acid, and 

 this exercises a powerful attraction upon the sperms 

 in response to which they quickly crowd into the 

 open archegonium and thus reach the egg within 

 it. A suitable solution of malic acid placed in a 

 fine glass tube will cause the free sperms to swim 

 into it much as they do into the open archegonium. 

 The investigations of Klebs have shown that in 

 many of the lower plants the character of the re- 

 productive cells may be largely controlled by the 

 nature of the medium in which the plants are grown. 

 Thus in a low alga, the water net (Hydro- 

 dictyon), the formation of the non-sexual repro- 

 ductive cells, zoospores, may be induced by culti- 

 vating the plants in a solution of maltose, while if 

 placed in a solution of cane-sugar, there is an ex- 

 traordinary development of the sexual reproductive 

 cells. In this connection we may also cite the in- 



