76 PLANT CELLS 



copper sulfate and narcotics accelerate the streaming movements. Light also 

 appears to increase the rate of streaming under certain conditions. 



The Physical Structure of Cytoplasm. — Any satisfactory explanation of 

 the structure of cytoplasm must account not only for its physical properties 

 and for its dynamic behavior, but must also explain how the innumerable 

 diverse ph3'sical and chemical reactions characteristic of living cells can occur 

 side by side in the same general medium. The highest magnifications reveal 

 no evidence of any structural background in active cytoplasm, yet its complex 

 activities suggest that it must possess an intricate structural organization. 

 The marked imbibitional capacity of cytoplasm, its stability toward electro- 

 lytes, its electrical properties, its viscosity, its coagulability and its gel-forming 

 capacity all suggest that it is to be classed with the hydrophilic colloids. All 

 modern students of cytoplasm are agreed that it is a hydrophilic colloidal sys- 

 tem of extreme complexity and variability. 



As was pointed out in the preceding chapter elastic gels are generally 

 considered to possess a submicroscopic structure of long interlacing fibrillar 

 units which hold a liquid phase in their irregular interstices. The same struc- 

 ture has been suggested for some hydrophilic sols. There is some evidence that 

 the structure of cytoplasm is similar. The cytoplasm in many living cells 

 appears fibrous and long delicate strands may extend through the vacuole of 

 the cell. Thread-like strands likewise appear in cytoplasm when it is being 

 manipulated with very fine glass needles. The sudden changes in viscosity 

 that living cytoplasm may undergo when disturbed, strongly suggest a fun- 

 damental structure similar to that of elastic gels. Furthermore, living proto- 

 plasm can itself flow slowly through small openings such as those in a fine 

 sieve without injury, but cannot be forced through somewhat larger holes 

 without serious injury or death. This behavior has been interpreted as evi- 

 dence that a fiber-like structure exists in protoplasm; the fibers being able 

 to slip through the holes when it is slowly streaming but being crushed and 

 destroyed if a mechanical pressure is utilized to force the protoplasmic mass 

 through the openings. 



On the basis of this and other evidence living cytoplasm is presumed by 

 some workers to have a skeletal structure of long submicroscopic fibrils. A 

 fluid phase in which are suspended the visible droplets and granules is be- 

 lieved to be held in the interstices between the fibrils. Solutes may be pre- 

 sumed to be present in both phases. The whole system of fibrils and liquid 

 is assumed to be dynamic and undergoing constant change, in which gel-like 

 fibrils are rapidly transformed into sols and the sols form fibrils with equal 

 rapidity. Such a structure will account satisfactorily for many of the prop- 

 erties of cytoplasm. For example, it offers an explanation for the rapid 



