314 HOW CROPS GROW. 
the seed should absorb water so that motion of the con- 
tents of the germ-cells can take place. Until the seed is 
more or less imbued with moisture, no signs of sprouting 
are manifested, and if a half-sprouted seod be allowed to 
dry the process of growth is effectually checked. 
The degree of moisture different seeds will endure or 
require is exceedingly various. The seeds of aquatic 
plants naturally germinate when immersed in water. The 
seeds of many land-plants, indeed, will quicken under wa- 
ter, but they germinate most healthfully when moist but 
not wet. Excess of water often causes the seed to rot. 
ce. Oxygen Gas.— Free Oxygen, as contained in the air, 
is likewise essential. Saussure demonstrated by experi- 
ment that proper germination is impossible in its absence, 
and cannot proceed in an atmosphere of other gases. As 
we shall presently see, the chemical activity of oxygen 
appears to be the means of exciting the growth of the 
embryo. 
d. Light.—It has been taught that light is prejudicial 
to germination, and that therefore seed must be covered. 
(Johnston’s Lectures on Ag. Chem. & Geology, 2d Eng. 
Eid., pp. 226 & 227). When, however, we consider that 
nature does not bury seeds but scatters them on the sur- 
face of the ground of forest and prairie, where they are, at 
the most, half-covered and by no means removed from the 
light, we cannot accept such a doctrine. The warm and 
moist forests of tropical regions, which, though shaded, 
are by no means dark, are covered with sprouting seeds, 
The gardener knows that the seeds of heaths, calceolarias, 
and some other ornamental plants, germinate best when 
uncovered, and the seeds of common agricultural plants 
will sprout when placed on moist sand or saw-dust, with 
apparently no less readiness than when buried out of sight. 
Finally, R. Hoffmann (Jahresbericht aber Agricultur 
Chem., 1864, p. 110) has found in experiments with 24 
