26 THE TROPISMS 



as regards utility or may be positively injurious, a consider- 

 able amount of adaptiveness is shown, especially to sub- 

 stances which are habitually encountered. The situation 

 is much as would be expected if starting with reactions which 

 primarily showed no relation to utility organisms came to 

 have their irritability modified and directed along service- 

 able lines to the extent, and only to the extent that was 

 necessary to insure survival under the conditions to which 

 they were naturally exposed. 



Chemotaxis is shown by parts of organisms, especially 

 the free cells such as spermatozoa, antherozoids and leuco- 

 cytes. Pfeffer showed that the spermatozoids of ferns 

 would collect hi capillary tubas containing a dilute solution 

 of malic acid and he concluded that the presence of this 

 substance in the archegonia is the means of drawing the 

 spermatozoids to the egg cell and thus effecting fertilization. 

 A similar r6le is supposed to be played by cane sugar in 

 mosses. The white blood cells which have the property of 

 crawling about much like Amoeba are attracted by certain 

 substances, and often gather in large numbers where there 

 is a bacterial infection. If a small tube containing a culture 

 of Staphylococcus aureus in agar is placed in one of the lymph 

 spaces of a frog the white corpuscles will migrate into it in 

 large numbers. A tube with the same culture medium, 

 but without the bacteria will not be invaded, showing that 

 it is the presence of some substance given off by the bacteria 

 which causes the leucocytes to enter the tube. 



The chemotactic movements of Amoeba and related 

 forms differ markedly from the scheme followed by the in- 

 fusoria and many flagellates. In Amoeba the part directly 

 affected is the first to act and there is a more or less definitely 

 directed series of movements away from the chemical. 

 In the positive reactions of Paramceeium there is no orienta- 



