OF VITAL FORCE IN GENERAL. 49 



regular rhythmical undulation; and the "zoospores" of the Confervece 

 are propelled through the water by the rapid vibration of the cilia with 

 which they are furnished ( 234). Now that such a production of a 

 purely physical change is a manifestation of vital force, is obvious from 

 this, that it takes place only while the vitality of the organism 

 endures, and that it is dependent upon the very same conditions as the 

 other vital operations require ; and it is further interesting to remark 

 in the case of the "zoospores," that it seems to take the place of the 

 operations of growth, for these do not commence until the movement of 

 the spore has ceased. The spiral filaments, again, which have been 

 discovered in most of the higher Cryptogamia, and which seem to per- 

 form the same function with the spermatozoa of Animals ( 240), have 

 a similar spontaneous movement, which must be looked upon as an ex- 

 pression of their vital force. Many cases of motion produced by a 

 change of form of certain contractile cells, might be cited from among 

 the higher tribes of the Vegetable kingdom ; these movements being 

 sometimes rhythmical and spontaneous, as in the Hedysarum gyrans, 

 sometimes taking place only in respondence to stimulation, as in the 

 Dionoea muscipula (Venus's fly-trap), and sometimes occurring as part 

 of the series of ordinary vital phenomena, although producible also by 

 stimulation, as in the Mimosa pudica (sensitive plant), which regularly 

 closes its leaves at night, but will do so at any time when they are 

 touched or otherwise irritated.* These movements only take place 

 during the life of the Plant ; and it is particularly observable in the 

 last-named species, that the facility with which they may be excited in 

 any individual is closely related to the activity of its vegetating pro- 

 cesses. Thus even in the Plant, we see that the Vital forces manifest 

 themselves, not merely in growth, but in movement. 



59. When we examine the structure of one of the higher Plants, we 

 find that, although the principal part of its fabric is still made up of 

 unmetamorphosed cells, yet that certain portions of it have undergone 

 histological transformation; that is, its primordial cells have lost their 

 original character, having been changed into other kinds of tissue. 

 This transformation takes place to a much greater extent in the Animal 

 body ; in which the variety of actions to be performed is much larger, 

 and in which we accordingly find a much greater variety of tissues de- 

 veloped as their instruments. But however widely these tissues may 

 depart from their original character, we find that the process of trans- 

 formation takes place under the same conditions as that of growth, and 

 must be regarded as a continuation of it ; being, in fact, the special 

 manifestation of vital force in one set of cells, as multiplication is in 

 another, or as motion in another. And we shall find, that, in proportion 

 as this transformation takes place, do the tissues lose their proper vital 

 endowments ; for it may be stated as a general fact, that even in the most 

 complicated and elaborate Animal organism, all the most active vital ope- 

 rations are performed by tissues which retain their original cellular con- 

 stitution with little or no change. Further, it is to be observed, that as it 

 is the peculiar character of such organisms that each of their parts should 



* For a fuller analysis of these phenomena, see the Author's "Principles of Physio- 

 logy* General and Comparative," CHAP. xix. 



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