DEATH OF PLANTS. 281 



ready to be absorbed by the plants ; proportionate, 

 in fact, to the augmented vigor of the vegetation. 



739. Hence, in endeavoring to introduce into one 

 country the plants of another, it is necessary to con- 

 sider the conditions under which they naturally 

 grow ; and care must be taken to imitate as closely 

 as possible those conditions, not merely as regards 

 soil, but likewise in respect to temperature, moisture, 

 and light. These, however, are practical matters, 

 into which it is unnecessary here to enter. 



740. In the last stage of the existence of a plant, 

 or when, either from excessive cold, disease, or merely 

 old age, the vitality becomes extinct, all that curious 

 series of changes by which organic matters are gene- 

 rated under the influence of light in the cells of 

 plants, ceases ; decay commences, the organic com- 

 pounds of the plant begin to decompose, and their 

 elements rearrange themselves into simpler forms. 

 The greater part of the hydrogen combines with 

 oxygen, and is gradually given off in the state of 

 water ; the nitrogen and hydrogen combine, and form 

 ammonia ; whilst the carbon is slowly dissipated in 

 the air in the state of carbonic acid, and at last little 

 remains beside mould, or charcoal in combination with 

 a small quantity of oxygen and hydrogen (673), and 

 the insoluble earthy matters which the plant may 

 have contained. 



741. A plant dies, when, at the end of the season, 

 it has passed through the various stages of its exist- 

 ence, fulfilled the office for which it was created, and 



24* 



■% 



