260 The Structure of Protoplasm 



fluid protoplasm is contractile. Usually the contracting regions of 

 a Plasmodium are of high viscosity, but protoplasm need not be so to 

 exhibit contractility. The significance of this remark is twofold: 

 It emphasizes that the chief feature of protoplasmic movement is not 

 a sol-gel transformation but the contractility of protoplasm, and 

 that organic colloidal solutions have some of the properties of gels 

 no matter how thin they may be. 



The physical differences between streaming protoplasm and quiet 

 protoplasm may be great, but they are purely physical and relative. 

 Quiet protoplasm is more viscous than flowing protoplasm; it may 

 be quite firm. It is usually more resilient, elastic, of higher tensile 

 strength, and more contractile than flowing protoplasm, but the 

 latter is by no means devoid of these properties. Fluid protoplasm 

 is elastic and contractile. The experiment cited on page 250, wherein 

 an elastic fluid was given a swish in a flask, is a demonstration of 

 contractility in a fluid system having a viscosity value but twice 

 that of water. The protoplasm which flows in the capillaries of a 

 slime mold is less contractile than the surrounding protoplasm 

 which is responsible for the flow, but it is, nevertheless, capable of 

 exhibiting contractility. 



Before turning from a purely biological discussion of rhythmic 

 contractility in protoplasm to a physical interpretation of it, I should 

 like to restate and further support the important concept that a Plas- 

 modium is an aggregation of centers of activity each with an indi- 

 viduality all its own. The physiological state of any one center 

 differs from that of adjoining centers at any given moment very 

 much as adjoining cells in tissue differ, with this exception, that in 

 a Plasmodium the difference is less permanent. 



Further indication that a slime mold is an assemblage of indi- 

 vidual units is to be had in the habit plasmodia have of breaking up 

 into microscopic bits at the time of sclerotium formation, and even 

 during the plasmodial stage. At sclerotium formation, the drying 

 Plasmodium fragments into many small sections, the whole resem- 

 bling an assemblage of cells. De Bary noticed this, and it has recently 

 been recorded for Hemitrichia vespariuvi by Ruth N. Nauss, who 

 describes the sclerotium as made of many isolated globules of resting 

 protoplasm. These subdivisions represent previous centers of activ- 

 ity which were physiologically distinct at the time the sclerotium 

 was formed. It is possible that they are uninucleate masses of proto- 

 plasm. The bits which are morphologically separate units in a 

 sclerotium cease to be so in a plasmodium where, however, they 



