20 SEEDS AND SEEDLINGS 



17. Water. — Seeds are usually rather dry ou issuing 

 from the fruit. Dryness makes the seed hardy. In 

 contact with water therefore, at the time of germination, 

 they often swell to two or three times their dry volume. 

 Actual growth in plants, too, always requires much water. 



18. Warmth. — Moderate heat has a strong influence in 

 hastening germination. For Indian Corn and Squash the 

 most favorable temperature is given as about 81° Fahr. 

 A few exceptional seeds will sprout at the freezing point 

 of water. Thus seeds of a Maple have been germinated 

 on a block of ice, the rootlets penetrating to a depth of 

 more than two inches into the dense, clear ice, in which 

 they melted out cylindrical cavities for themselves. The 

 requisite heat is here generated by the seedling itself. 



19. Oxygen is actively inhaled and combines with the 

 substances of the embryo. This oxidation furnishes energy 

 which appears in growth and in vital heat ; that is, in heat 

 in the seedling similar in all respects to the bodily warmth 

 of animals. 



20. As a result of oxidation carbonic acid gas is formed 

 and exhaled. The young plant thus breathes in and out. 

 Respiration is common to all living things. But in plants 

 the in-take of the one gas and the out-going of the other 

 are slow, continuous, and imperceptible processes. 



21. The development of seedlings. — If one looks under 

 the White Oak in late autumn, he is likely to find that the 

 acorns have sprouted. He will then discover that many of 

 the nuts, if lying on proper surface, for instance on short- 

 cropped pasture sward, are already fast-bound to the earth, 

 the radicles^ or incipient roots, having penetrated the soil. 

 It appears, therefore, that seeds may germinate and attach 

 themselves without being covered up ; though a covering 

 of some sort, as sand, soil, or dead leaves, is advantageous, 

 and some fruits, or their carpels, are even provided with 

 mechanical contrivances for partially burying themselves.^ 



22. Suppose that a seed lies thus, like the acorn, cleanly 

 upon the surface, and that it has been drenched by rain 



1 See Fig. 279. 



