OLIVE : MONOGEAPH OF THE ACKASIEAE. 
469 
induce such aggregation is evidently lacking, and this form might, 
therefore, he properly excluded from the true Acrasieae. It is true, 
however, that aggregations of the individuals of Sappinia do occur 
at the ends of straws or other small projections above the surface of 
the moist substratum ; but the amoebae, in my experience, fail to 
form colonies on a smooth surface, but instead, finally become en¬ 
cysted singly. Nevertheless, while these amoebae do not form 
aggregations either on nutrient agar tubes or on van Tieghem cell 
cultures, it does not perhaps, necessarily follow that certain individ¬ 
uals may not exert a chemotactic influence at certain periods, since 
analogous instances have been recorded among the Myxohacteriaceae^ 
in which colonies fail to form on agar surfaces. Sappinia, at any rate, 
is of special interest in that it might be regarded as a transitional 
form, in which the phenomena of fructification have advanced a 
step beyond those of the true Amoebae, but which has probably 
not attained to the differentiation seen in the lower Acrasieae. 
The force which causes the occasional accidental aggregations in the 
case of Sappinia is, in all probability, simply negative hydrotaxis. 
The same force is evidently operative in the formation of the iso¬ 
lated pedicelled cysts of this form, in which a single amoeba becomes 
erected at right angles to the substratum, thus producing a 
rounded or pear shaped body raised on a more or less slender stalk 
(pi. 5, fig. 2). 
In order to discover if possible the cause of the aggregation in 
the true Acrasieae, experiments were made with dilute solutions of 
some organic substances, with negative results. Pfeffer’s well 
known methods for investigating the chemotactic influences direct¬ 
ing the movements of spermatozoids were used in these experiments. 
Capillary tubes, about one eighth of an inch long and sealed at one 
end, were partially filled by means of an air pump with solutions of 
sugar and malic acid of various strengths. The tubes were then 
rinsed with distilled water and introduced into the hanging drop 
cultures of various species of Dictyostelium and Polysphondylium. 
No evidence whatever was gained as to any directive or repellent 
influence of these substances on the myxamoebae. At any rate, 
the substance which causes the contagious stimulus and exerts an 
attracting influence on the myxamoebae must evidently vary in 
different forms, since two well marked species of Dictyostelium, 
one for example, with white spores and another with dark, sown in 
