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 BischoflP, 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 Spiralium structura et functione Commentatio : 
Bonnae, 1829. It must be sufficient to state, that, by accu- 
rate chemical tests, by the most careful purification of the 
water employed from all presence of air, and by separating 
bundles of the spiral vessels of the gourd (Cucurbita Pepo), 
and of some other plants from the accompanying cellular 
substance, he came to the following conclusions, which, if not 
exactly, are probably 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- 
