OLIVE : MONOGKAPH OF THE ACRASIEAE. 
489 
Summary. 
The Acrasieae are naturally divisible into the Guttulinac^ae and 
the JDictyosteliaceae. These groups agree, first, in the fact that the 
individuals, during their vegetative state, assume the form of amoe¬ 
boid cells which never pass through a swarm cell condition, and 
secondly, in that the individuals, during the fructifying period, prob¬ 
ably in response to some chemotropic stimulus, unite to form 
colonies, called pseudoplasmodia, for the evident purpose of form¬ 
ing fructifications. 
On the other hand, they differ widely in the general character and 
structure of their fructifications. Those of the Guttidinaceae^ for 
example, possess little or no differentiation, while those of the Dictyd- 
steliaceae show considerable differentiation among the individuals, 
both in structure and in function. 
In the first mentioned group, the fructifications consist either of 
sessile, naked masses of pseudospores or spores, in which all of the 
individuals are similar in foi'm, or of stalked sori, in which there 
may usually be observed a small amount of differentiation, in that 
those individuals which, together with an enclosing mucous sub¬ 
stance, form the stalk, retain the elongated shape of the myxamoe- 
bae, instead of becoming oval or spherical bodies as do those indi¬ 
viduals in the sorus at the summit. 
In the higher group of the Dictyosteliaceae, the myxarnoebae of 
the fructifying pseudoplasmodium become either cells of the more 
or less highly differentiated stalk, or spores borne in a sorus at the 
summit of the stalk. In all cases, the differentiation consists essen¬ 
tially in the vacuolation of those individuals which are destined to 
take part in stalk formation. Large permanent vacuoles, usually 
one to each individual, are formed in a vertical column of myx- 
amoebae, which is centrally placed in the pseudoplasmodium, the num¬ 
ber of individuals which thus undergo differentiation being usually 
directly proportionate to the size of the aggregation. Each of the 
turgid vacuolated myxarnoebae of the stalk column ultimately 
secretes a cellulose wall, which becomes firmly cemented to the 
walls of surrounding individuals, and the mass of polygonal stalk 
cells thus formed, each with a large central vacuole and a small 
amount of peripheral protoplasm, closely resembles parenchymatous 
cells. 
