VEGETABLE PHYSIOLOGY. 6 



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hive their period of germination protracted till the temperature of 

 the latter is raised to that of the former. This is well exemplified 

 in the case of our green house and hot house plants, from which 

 it is also obvious that the temperature must not be raised beyond 

 a certain degree, otherwise the vital principle is totally destroyed. 



4. Access of Moisture. A fourth condition necessary to 

 germination is the access of moisture. Seeds will not germinate 

 if they are kept perfectly dry. Water, therefore, or some liquid 

 equivalent to it, is essential to germination. Hence rain is always 

 acceptable to the farmer or gardener, immediately after he has 

 sown his seeds ; and if no rain falls, recourse must be had, if 

 possible, to irrigation. But the quantity of water applied is not 

 a matter of indifference. There may be too little, or there may 

 be too much. If there is too little, the seed dies for want of 

 moisture ; if there is too much, it then rots. The case is not the 

 same, however, in all seeds. Some can bear but little moisture, 



.though others will germinate even when partially immersed, and 

 indeed there are some that will germinate when wholly submersed. 



5. Access of Atmospheric Air. A fifth condition necessary 

 to germination, is the access of atmospheric air. Seeds will 

 not germinate if placed in a vacuum. Ray introduced some 

 grains of lettuce seed into the receiver of an air pump, which he 

 then exhausted. The seeds did not germinate. But they 

 germinated upon the readmission of the air, which is thus proved 

 by consequence to be necessary to their germination. 



The discovery of the several gasses, and of their various 

 chemical properties, has contributed more than all other circum- 

 stances put together, to explain and elucidate the phenomena of 

 vegetation. The first experiments on this obscure but interesting 

 subject are those of Scheele ; who discovered soon after the 

 introduction of pneumatic chemistry, that beans did not germinate 

 in any kind of gas indifferently; but that oxygen gas is necessary 

 to the process. Achard afterwards proved that no seed will 

 germinate in nitrogen gas, or carbonic acid gas, or hydrogen gas, 

 except when mixed with a certain proportion of oxygen gas ; 

 and hence concluded that oxygen gas is necessary to the germi- 

 nation of all seeds, and the only constituent part of the atmospheric 

 air which is absolutely necessary. The experiments of JVI. 



