314 HOW CEOPS GROW. 



the seed should absorb water ^so tliat 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 seed 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. 



c. Oxyg'en Gas. — Free Oxygen, as contained in the au-, 

 is likewise essential. Saussure demonstrated by experi- 

 ment that proper germination is impossible in its absence, 

 and cannot proceed in an atmosj)here 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 Ziectures on Ag. Ghem. <& Geology, 2d Eng. 

 Ed., pp. 226 & 237). 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 (tTahresbericht ilber Agrieultur 

 Chem., 1864, p. 110) has found in experirqents ^yith 24 



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