THE CELL 



27 



Spallanzani, the same Italian priest who, questioning every- 

 thing, proved that life comes only from life, first (in 1766) 

 described the contractile vacuole of Protozoa (Fig. 25). A con- 

 tractile, or pulsating, vacuole is not usually found in plants. 

 The animal-like, free-swimming swarm spores of low forms of 

 plant life, such as algae and slime molds, possess contractile 

 vacuoles. For this reason, some of these lowly forms are con- 

 sidered to be of animal rather than of plant origin. 



Vacuoles may be classified under the following types: 



I. Nonpulsating 



A. Simple and not 

 rhythmical 



B. Complex and 

 rhythmical 



1. Permanent 



2. Wandering 



II. Pulsating 



A. Large central 



B. Small and 

 scattered 



1. Food 



2. Tannin 



3. Lecithin 



III. Doubtful forms 



A. Alveoli 



B. Golgi apparatus 



The foregoing classification is merely a temporary and con- 

 venient grouping to assist in a coherent discussion. The form, 

 structure, contents, mechanism, 

 origin, and function of vacuoles 

 differ greatly. The most com- 

 mon type is globular and non- 

 pulsating. In animal cells, this 

 type of vacuole is small, numer- 

 ous, and scattered, while in 

 plants, it is usually single, of 

 large size, and assumes a prom- 

 inent central position in the Fig. 18. — The filamentous type of 



mature plant cell. The contents X°'''(;Z''/.'VUt,t"' """' 

 of such vacuoles consist of salts, 



sugars, and nitrogenous (protein) matter in aqueous solution or 

 colloidal dispersion. Many one-celled animals contain a contrac- 

 tile vacuole. It may have a fixed position, as in the more highly 

 organized forms of Protozoa such as Vorticella and Paramecium, 

 or it may be of the wandering type, as in Amoeba. Less com- 

 mon is the reticulate, or netlike, vacuole which has recently been 

 brought to attention through the work of I. W. Bailey (Fig. 18). 

 This type of vacuole is relatively common in plant cells and may 

 possibly be identical with the Golgi apparatus (Fig. 5) in animal 



