418 CORRELATION OF PHYSICAL AND VITAL FORCES. 



existence stands in an inverse ratio to the energy of their ac- 

 tion ; the leaves of "evergreens," which are not cast off until 

 the appearance of a new succession, effecting their functional 

 changes at a much leas rapid rate than do those of " deci- 

 duous " trees, whose term of life is far more brief. 



Thus, the final cause or purpose of the whole Vital Activ- 

 ity of the Plant, so far as the individual is concerned, is to 

 produce an indefinite extension of the dense, woody, almost 

 inert, but permanent portions of the fabric, by the successional 

 development, decay, and renewal of the soft, active, and tran- 

 sitory cellular parenchyma ; and according to the principles 

 already stated, the descent of a portion of the materials of the 

 latter to the condition of binary compounds, which is mani- 

 fested in the largely-increased exhalation of carbonic acid 

 that takes place from the leaves in the later part of the sea- 

 son, comes to the aid of external Heat in supplying the force 

 by which another portion of those materials is raised to the 

 condition of organized tissue. The vital activity of the 

 Plant, however, is further manifested in the provision made 

 for the propagation of its race, by the production of the 

 germs of new individuals ; and here, again, we observe that 

 whilst a higher temperature is usually required for the devel- 

 opment of the flower, and the maturation of the seed, than 

 that which suffices to sustain the ordinary processes of vege- 

 tation, a special provision appears to be made in some in- 

 stances for the evolution of force in the sexual apparatus it- 

 self, by the retrograde metamorphosis of a portion of the or- 

 ganic compounds prepared by the previous nutritive opera- 

 tions. This seems the nearest approach presented in the 

 Vegetable organism, to what we shall find to be an ordinary 

 mode of activity in the Animal. That the performance of 

 the generative act involves an extraordinary expenditure of 

 vital force appears from this remarkable fact, that blossoms 

 which wither and die as soon as the ovules have been fertil- 

 ized, may be kept fresh for a long period if fertilization be 

 prevented. 



