VEGETABLE PHYSIOLOGY. 
383 
in pith, as in the Cassava ; in the disc of the flower, as in Amygdalus; 
and in bark, as in all exogenous plants; and cellular tissue is the 
principal or exclusive constituent of these. 
“ Woody Jibre is apparently destined merely for the conveyance of 
fluid upward or downward, from one end of a body to another, and for 
giving firmness and elasticity to every part. 
“ The real nature of the functions of the vascular system has been 
the subject of great difference of opinion, and may, indeed, be said to 
be so still. Spiral vessels have been most commonly supposed to be 
destined for the conveyance of air ; and it seems difficult to conceive 
how any one accustomed to anatomical observations, and who has 
remarked their dark appearance when lying in water, can doubt the 
fact. Nevertheless, many authors, and among them Dr. Dutrochet, 
assert that they serve for the transmission of fluids upward from the 
roots. This observer states, that c if the end of a branch be immersed 
in coloured fluid, it will ascend in both the spiral vessels and ducts ; 
but that, in the former, it will only rise up to the level of the fluid in 
which the branch is immersed; while, through the latter, it will travel 
into the extremities of the branches.’ It has, however, been asked 
with much justice, ‘ How the opinion that spiral vessels are the 
sap vessels is to be reconciled with the fact of their non-existence in 
multitudes of plants in which the sap circulates freely?’ To which 
might have, or perhaps has been, added the questions, f Why they do 
not exist in the wood, where a movement of sap chiefly takes place in 
exogenous trees ? ’ and also, f How it happens that their existence is 
almost constantly connected with the presence of sexes, if they be truly 
sap-vessels?’ And, further, it has always been remarked, that if a 
transverse section of a vine, for instance, or any other plant, be put 
under water, bubbles of air rise through the 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 coloured 
fluid, but denied in other circumstances. Indeed, to any observer ac¬ 
quainted with the difficulty of microscopic investigations, the obscurity 
that practically surrounds a question of. this sort must be apparent 
enough.” 
Dr. Bischoff is quoted by Dr. L. as having come to the following 
conclusion concerning the functions of spiral vessels, viz.: — “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, 
