20 A TEXTBOOK OF GENERAL BOTANY- 
If it were divided into cubes with sides 1 p (0.001 millimeter) 
long, there would be a total area of 6 square meters, while if 
the original cube were divided into cubes with edges 0.001 pu 
long, the total surface would be 6000 square meters. 
Hydration of protoplasm. The particles of many colloids have 
the property of absorbing and holding large quantities of water. 
This property is known as hydration, and a colloid which has 
absorbed water is said to be hydrated. Gelatin is a colloid and 
affords a good example. A 2 per cent solution of gelatin is a 
solid at ordinary temperature. Thus, two parts of gelatin can 
hold ninety-eight parts of water. The colloids of protoplasm are 
hydrated, the degree of hydration varying under different 
conditions. 
Distribution of protoplasm. The protoplasm is usually divided 
into two parts :-the nucleus, a rounded body; and the cytoplasm, 
which is the protoplasm outside of the nucleus. These are shown 
in the cells in Fig. 11. Young cells are usually filled with proto- 
plasm. In mature cells the cytoplasm may consist of a layer 
around the cell walls, a layer around the nucleus, and strands 
radiating from around the nucleus toward the cell walls (see 
Figs. 10, 11); or the cytoplasm may simply occur as a layer 
around the cell walls and the nucleus be embedded in this layer 
(Fig. 13). The nucleus is generally regarded as the part of the 
cell which governs the activities of the other parts and deter- 
mines the nature and hereditary characteristics of the plant. 
Inclusions within the protoplasm. Protoplasm consists essen- 
tially of a colloidal dispersion of proteins in water, but many 
other substances may be found within the protoplasm; these 
include mineral salts, sugar in solution, and food particles. 
Physiological properties of protoplasm. Protoplasmis frequently 
said to be distinguished from nonliving matter by the following 
physiological properties : 
1. Absorption and excretion. By absorption protoplasm obtains 
materials necessary for its growth, and by excretion it gets rid 
of some substances which it does not need. The protoplasm of 
green plants takes in water and mineral matter from the soil, 
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