68 Verfetahle Flujsiohujy. 



which exists in animal tissues, and they may, in all vegetables, 

 be readily referred to one type, of which they are very simple 

 modifications. 



This simplicity of vegetable structures, while it renders the 

 study of their anatomy more easy, makes tliis the more indis- 

 pensable to the vegetable physiologist ; since it is evident that 

 if the different external organs, such as the leaves, stems, and 

 roots, can all exercise any of the functions of vegetable life, the 

 general anatomy or study of external form can be of little use in 

 guiding us, and we must make ourselves acquainted with the 

 characteristics of the elementary tissues of which any given 

 organ is composed. 



To illustrate this, we are not liable to mistake when we say 

 that in Man and the higher animals respiration is performed 

 by the lungs. We could not say in the same general way that 

 the leaves constitute the respiratory organs of plants, for this 

 function is not only ordinarily performed in part by green shoots 

 of the stem, but in some cases, as in the Cacti, the leaves are 

 represented by hard spines, and the stem assumes entirely the 

 respiratory function ; and yet the Cactacea? belong to the 

 highest class of plants. Again, the stomach and intestinal canal 

 of animals in general are the organs for the absorption of food ; 

 and this function is only combined with others when the whole 

 organization is very low in the scale : but in plants we not un- 

 commonly see the roots assuming additional or different functions 

 even in the highest forms of vegetable life ; for in the turnip, 

 carrot, and other analogous cultivated plants, the root becomes 

 an organ not simply of absorption, but for the deposition and 

 •temporary preservation of assimilated food. In the ivy, tufts of 

 adventitious roots spring out from the stem to form merely me- 

 chanical organs of attachment to the bodies on which the plant 

 climbs ; while the constant occurrence of adventitious or acci- 

 dental roots, developed from the stem under the influence of 

 -peculiar circumstances, proves still more strikingly the modifi- 

 able character of the general organization of plants, even of those 

 standing highest in point of anatomical structure. 



Modification and change are Indeed the most striking attributes 

 of living objects, those which best mark their difference from and 

 pre-eminence over lifeless matter : and such being the case, it 

 must at once appear evident to every thinking mind that the study 

 of forms or conditions existing at any one point of time can lead 

 but a little way into the secrets of the laws of life. To trace 

 these to their converging points, to penetrate to the inner con- 

 nexion which exists in the midst of the multiformity of appear- 

 ances, it is necessary to follow step by step the gradual unfolding 



