250 PHYSIOLOGY. BOOK II. 



water from the mouths of the spiral vessels. But then, it has 

 been urged, that coloured fluids manifestly rise in the spiral 

 vessels ; a statement that has been admitted when the spiral 

 vessels are wounded at the part plunged in the colouring fluid, 

 but denied in other circumstances. Indeed, to any observer 

 acquainted with the difficulty of microscopic investigations, 

 the obscurity that practically surrounds a question of this sort 

 must be apparent enough. 



The subject has, however, been investigated with much 

 care by Bischoff', who instituted some very delicate and in- 

 genious experiments, for the purpose of determining the real 

 contents and office of the spiral vessels. It is impossible to 

 find room here for a detailed account of his experiments, for 

 which the reader is referred to his thesis, De vera Vasorum 

 Plantarum Sjnraliiim structura et functione Commentatio : 

 Bonnae, 1829. It must be sufficient to state, that, by accu- 

 rate chemical tests, by the most careful piirification of the 

 water employed from all presence of air, and by separating 

 bundles of the spiral vessels of the gourd [Cucurhita Pepo)^ 

 and of some other plants from the accompanying cellular 

 substance, he came to the following conclusions, which, if not 

 exactly, are pi'obably substantially, correct : — " That plants, 

 like all other living bodies, require, for the support of their 

 vital functions, a free communication with air ; and that it is 

 more especially oxygen, which, when absorbed by the roots 

 from the soil, renders the crude fluid fit for the nourishment 

 and support of a plant, just as blood is rendered fit for that 

 of animals. But, for this purpose, it is not sufficient that the 

 external surface should be surrounded by the atmosphere ; 

 other aeriferous organs are provided, in the form of spiral 

 vessels, which are placed internally, and convey air contain- 

 ing an unusual proportion of oxygen, which is obtained through 

 the root, by their own vital force, from the earth and water. 

 In a hundred parts of this air twenty-seven to thirty parts are 

 of oxygen, which is in part lost during the day by the surface 

 of plants under the direct influence of the solar rays." 



With such evidence of the aeriferous functions of the spiral 

 vessels it is difficult to contend ; and, indeed, it seems pro- 

 bable that this question is settled as far as spiral vessels, pro- 



