228 HOW CROPS GROW. 
thickening has a waved outline, and at points, the original cell-mem- 
brane is bare. Were these cells viewed entire, we should see at these 
points, on the exterior of the cell, dots or circles appearing like orifices, 
but being simply the unthickened portions of the cell-wall. The cells 
in fig. 32 exhibit each a central nucleus surrounded by grains of aleurone. 
Cell Contents. — Besides the protoplasm and nucleus, 
the cell usually contains a variety of bodies, which have 
been, indeed, noticed already as ingredients of the plant, 
but which may be here recapitulated. Many cells are al- 
together empty, and consist of nothing but the cell-wall. 
Such are found in the bark or epidermis of most plants, 
and often in the pith, and although they remain connected 
with the actually living parts, they have no proper life in 
themselves. 
All living or active cells are distended with liquid. This 
consists of water, which holds in solution gum, dextrin, 
inulin, the sugars, organic acids, and other less important 
vegetable principles, together with various salts, and 
constitutes the sap of the plant. In oil-plants, droplets of 
oil occupy certain cells, fig. 17, p. 90; while in numerous 
kinds of vegetation, colored and milky juices are found in 
certain spaces or channels between the cells. 
The water of the cell comes from the soil, as we shall 
hereafter see. The matters, which are dissolved in the sap 
or juices of the plant, together with the semi-solid proto- 
plasm, undergo transformations resulting in the production 
of solid substances. By observing the various parts of a 
plant at the successive stages of its development, under 
the microscope, we are able to trace within the cells the 
formation and growth of starch-grains, of crystalloid and 
granular bodies consisting chiefly of vegetable casein, and 
of the various matters which give color to leaves and 
flowers. 
The circumstances under which a cell developes deter- 
mine the character of its contents, according to laws that 
are hidden from our knowledge. The outer cells of the 
potato-tuber are incrusted with corky matter, the inner 
