Physiology of the Spermatozoa of Ferns. 5 71 
sperms, and Angiosperms. That the attraction was so 
general suggests that other substances beside malic acid 
compounds may have taken part in giving the results. 
Not every solution of mixed salts, one or more of which 
latter attract, is capable of giving an appreciable attraction. 
Thus with a nutrient solution for higher plants I could not 
determine an attraction at any concentration. The salts 
were present in the following proportions by weight: calcium 
nitrate 100, potassium nitrate 25, sodium chloride 15, magne¬ 
sium sulphate 50, and potassium phosphate (KH 2 P 0 4 ) 25. 
An attraction might have been expected from the presence 
of the potassium salts. Its absence was probably due to 
the acid reaction of the phosphate. It is doubtful whether 
the non-attracting calcium nitrate and sodium chloride affected 
the result. 
When the spermatozoa are liberated in distilled water 
1 could determine no attraction by tap-water or by water 
from a small pond. These weak salt solutions are not suf¬ 
ficiently concentrated to give any chemotactic effect, although 
individual salts which they contain (when more concentrated) 
certainly attract. In these experiments the prothallia were 
cleaned as thoroughly as possible in distilled water before 
the spermatozoa were liberated. 
Rubidium chloride was found to attract spermatozoa about 
as strongly as potassium chloride. Since other chlorides 
tested do not attract, the stimulus given by rubidium chloride 
appears to be due to the metal. Rubidium is a rare metal 
and only present in very minute quantities in the soil. When 
one reflects upon the kind of substratum upon which pro¬ 
thallia grow, it seems extremely improbable that rubidium 
salts ever gain access to the water-drops in which sperma¬ 
tozoa swim in sufficient quantity to cause attraction. We 
appear therefore to have, in my experiment with rubidium 
chloride, a case of a reaction to a stimulus without any 
special biological significance. Parallel cases seem to be 
given in the response of roots to the stimulus of light, electric 
currents and slow currents of water by alteration in the 
