CLASSIFICATION OF VITAL ACTIONS INTO FUNCTIONS. 221 



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is due. The great feature of the Nutritive operations in the Plant, is their 

 constructive character. They seem as if destined merely for the building-up 

 and extension of the fabric; and to this extension there may be no definite 

 limit. But it is very important to remark, that the growth of the more per- 

 manent parts of the structure is only attained by the continual development, 

 decay, and renewal of parts, whose existence is temporary. No fact is better 

 established in Vegetable Physiology, than the dependence of the formation of 

 wood upon the action of the leaves. It is in their cells, that those important 

 changes are effected in the sap, by which it is changed, from a crude watery 

 fluid containing very little solid matter, to a viscid substance including a great 

 variety of organic compounds, destined for the nutrition of the various tissues. 

 The "fall of the leaf" results merely from the death and decay of its tissue; 

 as is evident from the fact, that, for some time previously, its regular functions 

 cease, and that, instead of a fixation of carbon from the atmosphere, there is a 

 liberation of carbonic acid (a result of their decomposition) in large amount. 

 The process takes place in evergreens equally with deciduous trees; the only 

 difference being, that the leaves in the latter are all cast off and renewed to- 

 gether, whilst in the former they are continually being shed and replaced, a 

 few at a time. It appears as if the nutritious fluid of the higher Plants can 

 only be prepared by the agency of cells, whose duration is brief; for we have 

 no instance, in which the tissue concerned in its elaboration possesses more 

 than a very limited term of existence. But by its active vital operations, it 

 produces a fluid adapted for the nutrition of parts which are of a much more 

 solid and permanent character, and which undergo little change of any kind 

 subsequently to their complete development ; the want of tendency to decay 

 being the result of the very same peculiarity of constitution as that which 

 renders them unfit to participate in the proper vital phenomena of the organism. 

 Thus the final cause or purpose of all the Nutritive functions of the Plant, so 

 far as the individual is concerned, is to produce an indefinite extension of the 

 dense, woody, almost inert, and permanent portions of the fabric, by the con- 

 tinued development, decay, and renewal of the soft, active, and transitory 

 cellular parenchyma. The Nutritive functions, however, also supply the 

 materials for the continuance of the race, by the generation of new individuals; 

 since a new germ cannot be formed, any more than the parent structure can 

 be extended, without organizable materials, prepared by the assimilating pro- 

 cess, and supplied to the parts where active changes are going on. 



262. On analyzing the operations which take place in the Animal body, we 

 find that a large number of them are of essentially the same character with the 

 foregoing, and differ only in the conditions under which they are performed ; 

 so that we may, in fact, readily separate the Organic functions, which are 

 directly concerned in the development and maintenance of the fabric, from the 

 Animal functions, which render the individual conscious of external impres- 

 sions, and capable of executing spontaneous movements. The relative de- 

 velopment of the organs destined to these two purposes, differs considerably 

 in the several groups of Animals, as we have already in part seen (Chap. I.). 

 The life of a Zoophyte is upon the whole much more vegetative than animal ; 

 and we perceive in it, not merely the very feeble development of those powers 

 which are peculiar to the Animal kingdom, but also that tendency to indefinite 

 extension which is characteristic of the Plant. In the Insect we have the 

 opposite extreme; the most active powers of motion, and sensations of which 

 some (at least) are very acute, with a low development of the organs of nutri- 

 tion. In Man, and in the higher classes generally, we have less active powers 

 of locomotion, but a much greater variety of Animal powers ; and the instru- 

 ments of the organic or nutritive operations attain their highest development, 

 and their greatest degree of mutual dependence. "We see in the fabric of all 



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