PLANT-PRODUCTS STORES OF FORCE. 417 



verted into compounds of the latter kind by parting with some 

 of its carbon. But it may also take place at the expense of 

 compounds previously generated by the plant itself, and stored 

 up in its tissues ; of which we seem to have an example in 

 the unusual production of carbonic acid which takes place at 

 the period of flowering, especially in such plants as have a 

 fleshy disk, or receptacle containing a large quantity of starch ; 

 and thus, it may be surmised, an extra supply of force is pro- 

 vided for the maturation of those generative products whose 

 preparation seems to be the highest expression of the vital 

 power of the vegetable organism. 



The entire aggregate of organic compounds contained in 

 the vegetable tissues, then, may be considered as the expres- 

 sion, not merely of a certain amount of the material elements, 

 oxygen, hydrogen, carbon, and nitrogen, derived (directly or 

 indirectly) from the water, carbonic acid, and ammonia of the 

 atmosphere, but also of a certain amount of force which has 

 been exerted in raising these from the lower plane of simple 

 binary compounds, to the higher level of complex, " proxi- 

 mate principles ;" whilst the portion of these actually converted 

 into organized tissue may be considered as the expression of 

 a further measure of force, which, acting under the directive 

 agency of the germ, has served to build up the fabric in its 

 characteristic type. This constructive action goes on during 

 the whole Life of the Plant, which essentially manifests itself 

 either in the extension of the original fabric (to which in 

 many instances there seems no determinate limit), or in the 

 production of the germs of new and independent organisms. 

 It is interesting to remark that the development of the more 

 permanent parts involves the successional decay and renewal 

 of parts whose existence is temporary. The " fall of the 

 leaf" is the effect, not the cause, of the cessation of that pe- 

 culiar functional activity of its tissues, which consists in the 

 elaboration of the nutritive material required for the produc- 

 tion of wood. And it would seem as if the duration of their 

 18* 



