Differentiation 223 



behavior of these regions in regeneration. In iris leaves Weber ( 1941 ) 

 showed that the first indication of differentiation of stomatal mother cells 

 is a difference in osmotic concentration. 



Much cellular differentiation is due to changes in the cell wall. Boysen- 

 Jensen ( 1957 ) in a series of papers has demonstrated various wall changes 

 in the differentiation of root hairs with particular references to the action 

 of enzymes. 



These and many other observations show that morphological differen- 

 tiation has its physiological concomitants. To explain how these arise dur- 

 ing development is a major task of the student of differentiation. 



DIFFERENTIATION WITHOUT GROWTH 



There are a number of instances among the fungi where development 

 of the fruiting structures does not take place until the vegetative phase 

 of the life cycle has ended and no more food is absorbed from the en- 

 vironment. Growth, in the broader sense of the term, is therefore com- 

 pleted before differentiation begins, and the latter process can be studied 

 without the complications that are usually involved when growth accom- 

 panies it. One of the .most notable examples of this is furnished by the 

 Acrasiaceae, a family of the slime molds. 



The vegetative individual in these plants is a single amoeboid cell, or 

 7ni/xamoeba. These multiply profusely by simple division and live chiefly 

 on several species of bacteria. They can readily be grown in culture. After 

 vegetative life has gone on for some time and when external conditions 

 are favorable, a large number of these myxamoebae, in a group of from 

 several thousand to about 150,000, begin to move toward a center of 

 aggregation, streaming in from all sides and piling up into a mass of cells, 

 the pseud oplasmodium (Fig. 8-23). This is a millimeter or two in length, 

 is elongate in form, and somewhat resembles a small grub. It is sur- 

 rounded by a thin sheet of slime. By the time that this aggregation begins, 

 all vegetative growth has ceased, so that in the life cycle of these plants 

 growth (in the sense of increase by assimilation) and differentiation are 

 separate from each other in time. 



The process of aggregation seems to be controlled by the production 

 of a chemotactically active substance, acrasin. The timing and mechanism 

 of this process have been discussed by Shaffer ( 1957 ) . As to what deter- 

 mines the origin of these centers of aggregation, however, there is some 

 difference of opinion. Sussman (1952) believes that a few initiator cells 

 appear in the population and attract their neighbors into a many-celled 

 aggregate. 



Wilson (1952) presents evidence that aggregation has its origin in a 

 sexual process, two myxamoebae fusing early in aggregation and estab- 



