106 



HISTORY OF THE VEGETABLE KINGDOM. 



by a certain degree of humidity, accelerates the 

 evolution of the different parts of the embryo. 



Air is as useful to plants, in contributing to 

 their germination and growth, as it is necessary 

 to animals for respiration and the general func- 

 tions of life. Were a seed totally withdrawn 

 from contact with air, it would undergo no process 

 of development. Homberg, however, says he 

 got some seeds to germinate in the vacuum of 

 an airpump ; but although the experiment has 

 frequently been repeated, the same results have 

 never been obtained. It is, therefore, certain 

 that air is indispensably necessary for germina- 

 tion. Saussure, whose testimony is of such 

 weight in the experimental part of vegetable 

 physiology, is of opinion that Homberg's ex- 

 periments cannot in the least invalidate this 

 truth, and that the conclusions which he has 

 drawn from them must be considered as imper- 

 fect, and possessed of little accuracy. Seeds 

 buried too deeply in the ground, and thus with- 

 drawn from the action of atmospheric air, have 

 often remained for a very long time without ex- 

 liibiting any sign of life ; but when, by some 

 cause, they have been brought nearer the surface 

 of the ground, so as to come into contact with the 

 ambient air, their germination has been eifected. 



As air is not a simple body, but is formed of 

 oxygen and azote, it has been a subject of spec- 

 ulation whether both gases, or only one, are in- 

 fluential in the phenomena of germination. 



The action of air upon plants, at this first 

 period of their development, presents the same 

 circumstances as in the respiration of animals. 

 It is the oxygen of air that, in the act of re- 

 spiration, is the principal agent in giving the 

 blood the qualities which are to render it fitted 

 for the development of all the organs ; and the 

 same oxygen aids and facilitates the germination 

 of plants. Seeds placed in azotic gas or carbonic 

 acid gas are unable to genninate, and quickly 

 perish. We know that animals placed in similar 

 circumstances cease to respire, and die. But it 

 is not in a pure and separate state that oxygen 

 produces so favourable an effect upon the evolu- 

 tion of the germs. In this state it accelerates 

 germination at first, but soon puts a stop to it by 

 the too great activity which it communicates. 

 Accordingly, seeds, plants and animals, are unable 

 to genninate, respire or live, in pure oxygen 

 gas. Another substance must be mixed with it 

 to moderate its activity, before it can be rendered 

 fit for respiration and vegetation. It has been 

 found that a mixture of nitrogen or azote ren- 

 ders it better qxialified to perform this office, and 

 that the best proportions for the mixture are 

 one part of oxygen to three parts of azote or 

 nitrogen. 



The oxygen absorbed dming germination 

 combines with the excess of carbon which the 

 young plant contains, and forms carbonic acid. 



which is expelled. By this new combination, 

 the principles of the endosperm being no longer 

 the same, the fecula of which it is composed, 

 and which was insoluble before germination, 

 becomes soluble, and is often partly absorbed, 

 to afi^ord the first materials of nutrition to the 

 embryo. 



Certain substances appear to have a decided 

 influence in accelerating the germination of 

 plants, as we learn from the experiments of 

 Humboldt. That illustrious naturalist, to whom 

 almost every department of human knowledge is 

 indebted for some improvement, and many valua- 

 ble suggestions, has shown that the seeds of the cul- 

 tivated cress, when placed in a solution of chlorine, 

 germinate infiveorsix houi-s; whereas, if placed in 

 pure water, they would require thirty-six hours 

 to attain the same state. Certain exotic seeds, 

 which had resisted every method that had been 

 tried to make them germinate, became perfectly 

 developed in a solution of the same substance. 

 He farther observed, that all substances which 

 readily yield a part of their oxygen to water, 

 such as many metallic oxides, nitric and sul- 

 phuric acids sufficiently diluted, accelerated the 

 evolution of seeds, but at the same time pro- 

 duced the effect which we have remarked as re- 

 sulting from pure oxygen, that of exhausting 

 the young embryo, and quickly destroying its 

 vitality. 



Although seeds are usually placed in earth, to 

 germinate there, this circumstance is not abso- 

 lutely necessary for their development, as we 

 every day see seeds growing very well, and 

 with great rapidity, in fine sponges, or other 

 bodies which are kept soaked with water. But 

 let it not be imagined that earth is entirely use- 

 less or unnecessary for vegetation ; for the plant 

 extracts fi-om it, by its roots, substances which, 

 after converting them into nutritious elements, 

 it is enabled to assimilate. 



Light, so far from accelerating the develop- 

 ment of the organs of the embryo, retards it in 

 an evident manner. In fact, seeds always ger- 

 minate much more rapidly in darkness than 

 when exposed to the light of the sun. All seeds 

 do not take the same time in beginning to ger- 

 minate. Thus some seeds germinate in a very 

 short period: the cress in two days; spinach, 

 turnips, and kidney-beans in three days; the 

 lettuce in four ; melons and gourds in five ; most 

 of the gramineae in a week ; hyssop at the end 

 of a month ; others remain for a very long period 

 without showing signs of germination; some, 

 and chiefly those which have the episperm very 

 hard, or are surrounded by a woody endocarp, 

 germinate only at the end of a year ; while the 

 seeds of the hazel, the rose, the cornel, and others, 

 are not developed until two years after they ai-e 

 placed in the ground. 



The first visible effect of geraiination is the 



