The Seed and its Germination. 495 
kind is generally polygonal in shape, and consists of the 
following parts : (1) A cell wall, or limiting membrane, which 
separates it from its neighbours; (2) a layer of living sub- 
stance or protoplasm which lines the cell wall ; (3) a central 
cavity or vacuole. 
In young cells the cell wall is uniformly composed of cellulose, 
which is a material somewhat like starch in composition. It is 
homogeneous, transparent, capable of a certain amount of 
stretching, and possessed of some degree of elasticity ; capable 
also of permeation by water. The 
protoplasm, being the living con- 
stituent, is the essential part of the 
cell, and carries out all the vital 
processes. Besides lining the cell, 
it often extends across from side to 
side, forming bridles or bands, thus 
dividing up the vacuole or cavity 
into smaller spaces. In it is em- 
bedded, somewhere in its substance, 
a peculiar body called the nucleus. 
The protoplasm is like the cell 
wall in being expansible and some- 
what elastic, but differs from it 
in the degree of its permeability. 
Though water can pass through it 
readily, different substances in solu- 
tion in the water cannot do so with 
the same freedom. It has a very 
important function, therefore, in 
regulating the passage of various 
substances into and out of the cell. 
The central cavity contains a fluid 
known as the cell sap, which is composed of water holding in 
solution many different substances, as inorganic salts, sugar, 
colouring matters, vegetable acids, &c. By virtue of the pressure 
it exerts, it keeps the protoplasm in contact with the cell wall. 
The importance of the peculiar behaviour of the protoplasm 
just spoken of is evident when we reflect that, if it were freely 
permeable, the constituents of the cell sap could be easily washed 
out of the cells by prolonged soakage. Fig. 7 shows three 
vegetable cells. 
In such cells as these the nutritive materials for the use of 
the embryo are stored. All the different kinds of foodstuff 
which the living protoplasm requires for its nutrition and growth 
are represented bere, though in different proportions in different 
Fig. 7. — Three typical Vegetable 
Cells. 
a, cell-wall ; b, protoplasm, arranged 
partly as a circumferential layer, and 
partly in bands crossing the cell ; c 
nuckus; (/, vacuole, containing cell-sap 
