Comparative Physiology. 5Q5 



being deposited in the earth is found essential to produce per- 

 fect growth : the hyacinths grown in glasses are never found 

 to have the same luxuriance nor produce new roots in the same 

 perfection as those grown in the soil. The flagging of foliage 

 so perceptible in warm sultry weather has been generally 

 ascribed to increased exhalation, and has been found to cease 

 when the air gets suffused with moisture. By some it has been 

 referred to electricity, as a main cause ; and, as the same moisture 

 which assists absorption also conducts off electricity, it may 

 be difficult to say how far the opinion is right. The stomata 

 are allowed to be the breathing-pores for the inhalation and 

 exhalation of gaseous substances ; the exhalation of water is also 

 confined to the same organ, at least to the under surface of the 

 leaf, where they generally reside. As an illustration of change 

 of function, it is probable that when absorption is insufficiently 

 performed from the want of roots, or circumstances prevent 

 their efficiency, the stomata, when placed in contact with mois- 

 ture, perform the office of absorbents also. In such succulent 

 stems as Cacti this is probably a normal function which they 

 perform constantly, and the refreshing dews of the evening are 

 probably introduced principally through their instrumentality 

 in all. 



The next section, on Digestion and Absorption in Animals, is 

 very interesting ; indeed, the sections devoted to animals are in 

 all the functions the most interesting, from the complexity of 

 the function as it becomes more specialised ; but the intention 

 of the present essay being only to introduce as much of animal 

 physiology as will serve to illustrate that of vegetables, this will 

 be found sufficiently done in the extracts from the functions 

 considered generally. For those who wish to make themselves 

 masters of the details, and to have more than an outline of the 

 subject, which is all that can be done in an essay, the book itself 

 will be indispensable. 



In Chap. VI., On the Circulation of the Nutritive Fluid 

 generally considered, he remarks : " In beings of the most 

 simple organisation, where the materials of the tissues are sup- 

 plied throughout the whole surface by the constant permeation 

 of external nutriment, no transmission of fluid from one portion 

 of the system to another seems necessary ; and accordingly wc 

 And no evidence of it, either in any structure set apart, or 

 in any visible motion of the fluid. In more complex organ- 

 isms, however, where a small part only of the surface is 

 appropriated to absorption,' it becomes necessary that means 

 should be provided of conveying to distant parts the nutri- 

 ment they require. This is effected by the circulation of the 

 fluid absorbed through vessels and passages adapted to tliat 

 purpose, and its developement is proportional to the limitation 



