418 CORRELATION OF PHYSICAL AND VITAL FORCES. 



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

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

 the appearance of a new succession, effecting their functional 

 changes at a much less rapid rate than do those of &quot; deci 

 duous &quot; 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. 



