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 in capillary tubes 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 réle is supposed to be played by cane sugar in 
mosses. The white blood cells which have the property of 
crawling about much like Ameba 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 Amceba and related 
forms differ markedly from the scheme followed by the in- 
fusoria and many flagellates. In Amceba 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 Paramcecium there is no orienta- 
