EFFECT OF AIR AND LIGHT ON GERMINATION. • 131 



The degree of warmth which is required in order that seeds may be- 

 gin to grow, varies with the nature of tlie seed itself. In Northern Si- 

 beria and other icy countries, plants are observed to spring up at a tem- 

 perature but slightly raised above the freezing point (32° F.,) but it is 

 familiar to every practical agriculturist, that the seeds he yearly con- 

 signs to the soil require to be protected from the inclemency of the 

 weather, and sprout most quickly when they are stimulated by the 

 warmth of approaching spring, or by the heat of a summer's sun. 



The same fact is familiarly shown in the malting of barley, where 

 large heaps of grain are moistened in a warm atmosphere. When ger- 

 mination commences, the grain heats spontaneously, and the growth 

 increases in rapidity as the heap of corn attains a higher temperature. 

 It thus appears that some portion of that heat which the growth of the 

 germ and radicles requires, is provided by natural processes in the grain 

 itself; in some such way as, in the bodies of animals, a constant supply 

 of heat is kept up by the vital processes — by which supply the cooling 

 effect of the surrounding air is continually counteracted. 



We have seen in the preceding lecture, that the transformations of 

 which starch and gum are susceptible, take place with greater certainty 

 and rapidity under the influence of an elevated temperature. It will 

 presently appear that such transformations are also affected during ger- 

 mination ; there is reason, therefore, to believe that the external warmth 

 which is required in order that germination may begin, as well as the 

 internal heat naturally developed as germination advances, are both 

 employed in effecting these transformations. And, as the young sprout 

 shoots more rapidly under the influence of a tropical sun, it is probable 

 that those natural agencies in general, by which such chemical transfor- 

 mations are most rapidly promoted, are also those by which the pro- 

 gress of vegetation is in the greatest degree hastened and promoted. 



3°. It has been observed that seeds refuse to germinate if they are en- 

 tirely excluded from the air. Hence seeds which are buried beneath 

 such a depth of soil that the atmospheric air cannot reach them, will 

 remain long unchanged, evincing no signs of life — and yet, when turned 

 up or brought near tiie surface, will speedily begin to sprout. Thus in 

 trenching the land, or in digging deep ditches and drains, the farmer is 

 often surprised to find the earth, thrown up from a depth of many feet, 

 become covered with young plants, of species long extirpated from or 

 but rarely seen in his cultivated fields. 



4°. Yet light is, generally speaking, prejudicial to germination. 

 Hence the necessity o^ covering the seed, when sown in our gardens and 

 corn fields, and yet of not so far burying it that the air shall be excluded. 

 In the usual method of sowing broad-cast, much of the grain, even after 

 harrowing, remains uncovered : and the prejudicial influence of light in 

 preventing the healthful germination of such seeds is no doubt one rea- 

 son why, by the method of dibbling, fewer seeds are observed to fail, and 

 an equal return of corn is obtained from a much smaller expenditure of 

 seed. 



The reason why light is prejudicial to germination, as well as why 

 the presence of atmospheric air is necessary, will appear from the fol- 

 lowing observation : — 



5*^. When seeds are mads o germinate in a limited portion of atmos- 



