386 Comparative Physiology. 



their roots, stem, and leaves ; and, with the exception of the 

 organs of fructification, transformed from leaves in some cases, 

 present merely a repetition of similar parts, in all of which the 

 simple relation of branches to leaves is the same. A conse- 

 quence of this is, that each of these parts has the power of 

 becoming in its turn an independent living body ; the seed dif- 

 fering from the shoot only in its greater vegetative power." 

 Respiration, he says, affords a very important distinctive cha- 

 racter between animals and plants ; being performed in plants 

 by the whole surface, and in animals confined to an organ which, 

 in a small space, afibrds an immense surface for contact with the 

 atmosphere. The difference, however, is more structural than 

 functional: plants give off, or expire, carbonic acid constantly, 

 the same as animals do, though not in so great a quantity ; the 

 inhalation of oxygen and exhalation of carbonic acid form a con- 

 stant function, and seem necessary to their existence. The 

 whole surface, in the dark parts, is said to be capable of this 

 function in some degree, but in the higher classes of plants it is 

 principally confined to the leaves ; and the surface of animals is 

 said not to be wholly destitute of this power, even in the more 

 perfect. It is in the inhalation of carbonic acid by the green 

 parts of leaves, and the exhalation of oxygen, that plants differ 

 most from animals. This function is totally different from any 

 thing to be found in animals : it has been called digestion, but 

 seems totally different ; and would appear rather a distinct pro- 

 cess, necessary to plants only from the greater chemical action 

 required to prepare organised products from inorganic sub- 

 stances. The heat and light of the sunbeam being necessary to 

 perfect the great organic chemical action required, the organs 

 adapted to this function require to be developed externally. 



The most remarkable similarity subsists between plants and 

 animals, Miiller says, in the process of the developement of their 

 tissues. " The observations of Mirbel had shown that all the 

 forms of vegetable tissue are developed from cells, which at first 

 constitute the whole mass of the tissue, but afterwards undergo 

 various changes in their shape and size, so as to be converted 

 into woody fibre, spiral vessels, &c. M. Schleiden has more 

 recently traced the developement of the vegetable tissue at a 

 still earlier stage. The abundant gum of nascent parts of plants, 

 such as the youngest albumen of a seed, when examined by the 

 microscope, is seen to be turbid from the presence of minute 

 molecules : soon larger granules are also observed in it ; around 

 these granules, by a kind of coagulation, larger bodies are 

 formed, the cytohlasts, in which the above-mentioned granules 

 are still visible as nuclei. When the cytoblast has attained its 

 full size, a small vesicle appears on it ; this enlarges and becomes 

 the cell, in which the cytoblast is for a period still visible, either 



