472 Physiology 



The vacuolar cycle 



In the simplest cases, small vacuoles appear in the cytoplasm and 

 fuse to form a new vacuole which increases in volume (diastole) and then 

 collapses (systole) in discharging its contents to the outside. Canal-fed 

 vacuoles receive fluid during diastole from feeder canals which may per- 

 sist throughout the cycle. As described by Lloyd and Beattie (320) in 

 Paramecium catidatum, diastole involves: (1) an early rapid phase, coin- 

 ciding with contraction of the canals to force fluid into the vacuole; and 

 (2) a slow phase, in which further distension involves diffusion of water 

 into the vacuole from the cytoplasm. In systole there are: (1) a prelim- 

 inary slow phase, in which fluid passes from the vacuole into the canals, 

 distending them; and (2) a rapid phase, in which the remaining fluid is 

 expelled from the vacuole to the outside. Fluid of relatively high osmotic 

 pressure — that derived from the vacuole at the beginning of systole — 

 supposedly remains in the canals and facilitates withdrawal of water from 

 the cytoplasm in the next cycle. On the other hand, Gelei (146) believed 

 that connections between the vacuole and the canals are closed before 

 systole. This is also the case in Paramecium multimicronucleatum (294). 



The frequency of pulsation, in general, is gieater in fresh-water species 

 than in marine or parasitic forms. Cycles range from 6 seconds to 20 

 minutes for fresh-water species, 45 seconds to 32 minutes for marine and 

 brackish water types, and 72 seconds to 16 minutes for endoparasitic 

 forms (296). Fresh-water species eliminate a volume of water equivalent 

 to body volume in 4-45 minutes, whereas marine ciliates require 2.75-4.75 

 hours. In a given species, frequency of pulsation increases as the tempera- 

 ture rises within non-injurious limits. Temperature characteristics ([jl 

 values), calculated from the equation of Arrhenius, have been reported 

 for Spirostomum ambiguum, Blepliarisma undulans, and four species 

 of Paramecium over the range, 16-26.8° (145). 



According to the osmotic theory of diastole, water passes into the con- 

 tractile vacuole by osmosis from the cytoplasm. This mechanism would 

 require an osmotic gradient favoring the contents of the contractile 

 vacuole. Since it is not clear just how such a gradient would be main- 

 tained, it is difficult to account for diastole on this basis alone (296). The 

 filtration theory (129, 448) holds that hydrostatic pressure forces water 

 through the vacuolar membrane. Haye (195) and Kitching (296) have 

 pointed out that hydrostatic pressure would not be relieved by passage of 

 water into the contractile vacuole, since this organelle is surrounded by 

 cytoplasm during diastole. The secretion theory, favored by Kitching 

 (296), postulates secretion of water into the contractile vacuole by the 

 membrane. This assumption seems logical enough and it conflicts with no 

 available data. 



The discharge of the contractile vacuole has been explained in two 



