Book I. 



GERMINATION OF THE SEED. 



159 



into the receiver of an air-pump, which he then exhausted. The seeds did not germinate. 

 But they germinated upon the re-admission of the air, which is thus proved by conse- 

 quence to be necessary to their germination. Achard proved that no seed will germinate 

 in nitrogen gas, or carbonic acid gas, or hydrogen gas, except when mixed with a cer- 

 tain proportion of oxygen gas ; and hence concluded that oxygen gas is necessary to 

 the germination of all seeds, and the only constituent part of the atmospheric air which 

 is absolutely necessary. Humboldt found that the process of germination is accelerated 

 by means of previously steeping the seed in water impregnated with oxymuriatic acid. 

 Cress-seed treated in this manner germinated in the space of three hours, though its or- 

 dinary period of germination is not less than thirty-two hours. 



722. The period necessary to complete the jrrocess of germination is not the same in all 

 seeds, even when all the necessary conditions have been furnished. Some species require 

 a shorter, and others a longer period. The grasses are among the number of those plants 

 whose seeds are of the most rapid germination ; then perhaps cruciform plants ; then le- 

 guminous plants ; then labiate plants ; then umbelliferous plants ; and in the last order 

 rosaceous plants, whose seeds germinate the slowest. The following table indicates the 

 periods of the germination of a considerable variety of seeds, as observed by Adanson : 



Wheat, Millet -seed 

 Spinage, Beans, Mustard 

 Lettuce, Aniseed 

 Melon, Cucumber, Cress- 1 

 seed - - - J 



Days. 



3 

 4 



Radish, Beet -root 

 Barley 

 Orache 



Purslaia - .- 

 Cabbage - 



Days. 

 Hyssop .... 30 

 Parsley - - - 40 or 50 



Almond, Chestnut, Peach - 1 year 

 Rose, Hawthorn, Filbert - 2 years. 



723. Physical phenomena. When a seed is committed to the soil under the conditions 

 that have been just specified, the first infallible symptom of germination is to be deduced 

 from the prolongation of the radicle (Jig. 56. a) , bursting through its proper integuments, 



and directing its extremity downwards into the soil. The next step in the process of ger- 

 mination is the evolution of the cotyledon or cotyledons (c), unless the seed is altogether 

 acotyledonous, or the cotyledons hypogean, as in the oak (b). The next step, in the case 

 of seeds furnished with cotyledons, is that of the extrication of the plumelet (c), or first 

 real leaf, from within or from between the cotyledon or cotyledons, and its expansion in 

 the open air. The last and concluding step is the developement of the rudiments of a 

 stem (rf), if the species is furnished with a stem, and the plant is complete. Whatever 

 wav the seed may be deposited, the invincible tendency of the radicle is to descend and 

 fix itself in the earth; and of the plumelet to ascend into the air. Many conjectures 

 have been offered to account for this. Knight accounts for it on the old but revived 

 principle of gravitation. Keith conjectures that it takes place from a power inherent in 

 the vegetable subject, analogous to what we call instinct in the animal subject, infallibly 

 directing it to the situation best suited to the acquisition of nutriment and consequent de- 

 velopement of its parts. 



724. The chemical phenomena of germination consist chiefly in the changes that are effected in the 

 nutriment destined for the support and developement of the embryo till it is converted into a plant. 

 This nutriment either passes through the cotvledons, or is contained in them ; because the embryo dies 



