234 PROTOPLASM 



A number of interesting facts pertaining to problems other 

 than those bearing on elasticity as such, e.g., permeability 

 (see Fig. 47), are revealed by stretching cells. 



Nuclei may likewise be stretched. When isolated, a nucleus 

 is always abnormal, if not dead. The nucleoplasm is then often 

 very extensile, though it may tear like butter (Fig. 48). When 

 the nuclear plasm is elastic, it is probably alive; and when 

 inelastic, it is coagulated and dead. 



The elasticity of protoplasm varies greatly. It may be 

 artificially changed by the addition of salts (page 448). The 

 protoplasm of an onion cell which has rested overnight in sodium 

 nitrate has an elastic limit much lower than normal, while 

 protoplasm treated with a calcium salt is considerably more 

 elastic than that of the control cell. Magnesium has no effect. 

 Acid lowers the elastic (stretching) limit. Sodium usually 

 decreases viscosity and elasticity, while calcium increases both 

 properties, but acid increases viscosity (coagulates) and decreases 

 elasticity (extensibility). 



The value of elasticity measurements of protoplasm lies in 

 the fact that stretch is one of the few mechanical properties of 

 protoplasm that can be determined with fair accuracy and thus 

 serve as a reliable indicator of the effects of reagents and of 

 other experimentally induced changes in protoplasm. Elasticity 

 is also that property of protoplasm which gives the best indica- 

 tion that we have of the finer structure of living matter. As 

 Scarth says, elasticity and the other jelly properties of protoplasm 

 afford "the only available basis for conjecture as to its ultra- 

 microscopic structure," and McBain is of the opinion that the 

 presence of elasticity in a colloidal solution is a specific and posi- 

 tive test for the presence of ramifying aggregates. Continuity 

 in the structure of protoplasm is an important physical concept 

 about which we shall have more to say later; one evidence for 

 it lies in elastic properties. 



Most colloidal hypotheses of muscle action involve a considera- 

 tion of elasticity. H. J. Jordan makes elastic and structural 

 qualities the basis of his interpretation of the behavior of snail 

 muscle. This muscle may be stretched a short distance and 

 show perfect return (elasticity), but when elongated beyond 

 a certain point, there is no contraction whatever; the elastic 

 Hmit has been exceeded, causing a breakdown in structure. 



