220 On the Structure and Functions of the Organs of Respiration. 



effects produced by the whole. In the lowest classes of plants, we find a 

 very simple organisation, which is capable of effecting all that is necessary 

 for the maintenance of the individual and the continuance of the species. 

 In the algae and fungi, for instance, the structure is entirely cellular ; no 

 rudiments of vessels or traces of a general circulation are found in them j 

 and although in some of the algpe there seems a tendency in particular 

 parts to expand into leaves or leaf-like members, it cannot be said that 

 there is any portion specially adapted for the exposure of the fluids of the 

 plant to the influence of the air, the act of respiration taking place equally 

 throughout the whole surface. In mosses we find the first indications of a 

 higher form of organisation ; for in these plants there are distinct leafy 

 expansions j and in some species of liverwort fmarchantia poh/morphaj , 

 slomata, or minute openings in the cuticle specially formed for the passage 

 of air, have been observed by Mirbel and others. We also find amongst 

 mosses, the rudiments of tubular vessels, to which the elongated cells of the 

 chara form a beautiful transition j so that in the higher forms of these 

 plants we may trace an approach to a more elevated type, in the specializa- 

 tion of the respiratory and circulating apparatus. In the ferns there is an 

 extensive development of true leaves, plentifully furnished with stoniata ; 

 the tubular apparatus for the circulation of the fluids is more complete, 

 and we find the rudiments of spiral vessels or trachece, which in their 

 perfect state are confined to flowering plants. In endogens and exogens, 

 the organs adapted for the aeration of the sap are arrived at their highest 

 degree of perfection. A regular circulation of that fluid takes place, by 

 which every part of it is brought into communication with the air, 

 through the medium of the thin cuticle of the leaf, beneath which it rami- 

 fies ; and there is reason to believe that the stoniata have a tolerably direct 

 communication with the spiral vessels which are found in the midst of the 

 stem, and which appear, from the quantity of oxygen they contain, to have 

 an important relation with the function of respiration. It will hereafter 

 be seen that the cuticle of the green parts of plants is to be considered as 

 a digestive, as well as a respiratory surface. 



In the animal kingdom we find respiration exerting a more evident, 

 though perhaps not in reality a more powerful, influence over the system 

 than in vegetables. The necessity for air Is more urgent than the demand 

 for food; many animals being capable of subsisting a considerable time 

 without nourishment, bat all speedily perishing when deprived of air; and 

 a very remarkable correspondence may be observed between the activity 

 of this function in any Individual, and its general vital energy, which is 

 especially manifested by the development of its organs of locomotion, and 

 the degree of heat which it Is capable of maintaining. In illustration of 

 this fact, we need only refer to the comparative structure of the respiratory 

 organs in the active and rapacious eagle, and the timid and indolent tor- 

 toise. In making such comparisons, we must bear in mind that the extent 



