132 , SEEDb SPROUT 0]NLY IN THE TKESENCE OF OXYGEN. 



pheric air, the bulk of the air undergoes no material alteration, but on 

 examination its oxygen is found to liave diminished, and carbonic acid 

 to have taken its place. Tlierefore, during germination, seeds absorb 

 oxygen gas and give off carbonic acid. 



Hence it is easy to understand why the presence of air is necessary 

 to germination, and why seeds refuse to sprout in hydrogen, nitrogen, 

 or carbonic acid gases. They cannot sprout unless oxygen be within 

 their reach. 



We have seen also in a previous lecture that the leaves of plants in 

 the sunshine give off oxygen gas and absorb carbonic acid, — while in 

 the dark the reverse takes place. So it is with seeds which have begun 

 to germinate. When exposed to the light they give off oxygen instead 

 of carbonic acid, and thus the natural process is reversed. But it is ne- 

 cessary to the growth of tlie young germ, that oxygen should be absorb- 

 ed, and -carbonic acid given of — and as tliis can take place to the requir- 

 ed extent only in the dark, the cause of the prejudicial action of light is 

 sufficiently apparent as well as the propriety of covering the seed with a 

 thin layer of soil. 



6°. JDuring germination, vinegar (acetic acid) and diastase are pro- 

 duced. That such is the case in regard to the latter substance, has been 

 proved in the j)revious lecture, (p. 118.) That acetic acid is formed is 

 shown by causing seeds to germinate in powdered chalk or carbonate of 

 lime, when after a time acetate of lime* may be washed out from the 

 chalk (Braconnot) in whicli they have been made to grow. The acid 

 contained in this acetate must have been formed in the seed, and after- 

 wards excreted or thrown out into the soil. 



7°. When the germ has shot out from the seed and attained to a sen- 

 sible length, it is Ibund to be possessed of a sweet taste. This taste is 

 owing to the presence of grape sugar in the sap which has already be^ 

 gun to circulate through its vessels. 



It has not been clearly ascertained whether the vinegar or the dias- 

 tase is first produced when germination commences, but there seems 

 little doubt that the grape sugar is formed subsequently to the appejir- 

 ance of both. 



8^. The young shoot which rises upwards from the seed consists of 

 a mass of vessels, which gradually increase in length, and after a short 

 time expand into the first true leaves. The vessels of this first shoot do 

 not consist of unmixed woody fibre. It is even said that no true wood 

 is formed till the first true leaves are developed. — [Lindley's Theory of 

 Horticulture.] The vessels of the young s[)rout, therefore, and of the 

 early radicles, probably consist of the cellular Jibre of Payen. They 

 are uncjuestionably formed of a substance which is in a state of transition 

 between starch or sugar and woody fibre, and which has a constitution 

 analogous! ^^ that of both. 



Having thus glanced at the phenomena which attend upon germina- 

 tion, let us now consider the chemical changes by which these phenom- 

 ena are accompanied. 



1°. The seed absorbs oxygen and gives off carbonic acid. We have 



* Acetate of lime is a compound of acetic acid (vinegar) and lime, and may be prepared by 

 dissolving chalk in vinegar. It is very soluble in water, 

 t By analogous I mean vihich may be represented by carbon and water, 



