the entire cell volume (Figure 3-33). The contractile vacuoles of lower 

 forms appear to serve an excretory function, while the large ones of 

 the conducting tissues of plants may provide necessary turgor for sup- 

 port. Most evidence would suggest that vacuoles simply provide a suit- 

 able substrate for metabolic reactions. In any event, they seem to be of 

 universal occurrence in both plant and animal cells. 



Figure 3-33. Photomicrograph of Large Vacuole in Microspore Cell of 

 Tradescantia paludosa. Note eccentric position of vacuole and nucleus con- 

 taining two nucleoli. (From Leak, L. V. and Wilson, G. B., 1960. "Relative 

 Volume Changes of the Nucleolus in Relation to Cell and Nucleus in Pisiiin 

 sativum and Tradescantia paludosa," Trans. Am. Microscop. Soc, 79, Fig. 8, 

 p. 157. Courtesy of Mr. L. V. Leak, Michigan State University.) 



STRUCTURE AND FUNCTION OF CYTOPLASMIC ORGANELLES / 67 



