THE CELL 29 



most likely ones are the maintenance (in plant cells) and .the 

 regulation (in Protozoa) of osmotic (solution) pressure; storage 

 (in plant cells); and excretion. There can be no question that 

 the first of these functions is fulfilled by the large central vacuole 

 of plant cells which maintains the turgidity (osmotic pressure) of 

 cells. In this connection, it is interesting that fresh-water 

 Protozoa possess vacuoles while marine Protozoa do not, but 

 marine Protozoa which have become accustomed to fresh water 

 acquire vacuoles, while fresh-water Protozoa accustomed to salt- 

 water lose their vacuoles. The function of food vacuoles is 

 equally definite. They are formed when food is engulfed by 

 Protozoa. The function customarily ascribed to the contractile 

 vacuole of Protozoa is that of excretion through the discharge of 

 waste products ; in other words, the vacuole is crudely comparable 

 to the kidney of higher animals. 



Minute protoplasmic alveoli, little sacks or cavities which often 

 assume great regularity in form (Fig. 115), are also vacuoles of a 

 kind. C. V. Taylor has suggested this for Euplotes. He has 

 seen an alveolus from a disintegrating Euplotes float out into 

 the water, maintain its identity while swelling like a true osmotic 

 (solution pressure) system, and ultimately burst. 



C. J. Chamberlain has said that all types of vacuolization in 

 plant protoplasm, from the most minute globules to the large 

 central vacuole, are but forms of the same thing. The trans- 

 formation of spherical vacuoles into threadlike ones is evidence 

 of this. But it is equally true that the contractile vacuole of 

 Protozoa is a structure different, if not in function then at least 

 in its mechanism, from the simple, nonpulsating vacuole typical 

 of protoplasm in general, especially of plant cells. 



The Size of Cells. — Cells are mostly microscopic, though some 

 are visible to the naked eye, and some few quite large, relatively. 

 Certain one-celled organisms are large enough to be seen without 

 a lens, though they are then mere specks. Pine cambium cells 

 may be 2,000 ji (2 mm) long; and cells of the alga Nitella may be 

 several inches in length (Fig. 33). The one-celled alga Valonia, 

 which is spherical in form, may be half an inch in diameter. 

 Cells which build up tissues are usually of microscopic dimensions. 

 The Elodea leaf cell shown in Fig. 4 is 1^50 in. long, about one- 

 quarter as wide, and one-eighth as deep. Expressed in milli- 

 meters, such a cell is 0.15 by 0.04 by 0.03 mm. Expressed in 



