VEGETABLE PHYSIOLOGY: ELEMENTARY ORGANS. 
15 
lary rays which convey the elaborated fluid from the bark to the 
centre of the stem, and the parenchyma in which the sap is 
diffused on entering the leaves, are both composed of it: nearly 
all the bark, in which the downward current of the sap takes 
place, is also cellular ; and in endogenous plants, where no true 
bark exists, there is no other course for the descending sap than 
the cellular substance in which the vascular system is imbedded; 
it is therefore permeable to fluids, though it does not possess 
visible pores. It is the substance from which the leaf-buds are 
generated, for these organs always originate from the medullary 
system, of which it is the entire component. Fertilisation ap¬ 
pears to take place exclusively through its agency, for the coats 
of the anther, the tissue of the stigma, the ovules with their 
sacs, and the pollen itself, are all cellular, the vivifying particles 
of the latter being composed of the same kind of matter in a 
more minutely divided state. It is, moreover, the only tissue 
in which saccharine and amylaceous secretions are formed. 
When collected into hollow cylinders, as vasiform tissue, it 
serves for the more rapid transmission of fluids in the direction 
of the stems ; and the size of these is usually in proportion to 
the distance to which it has to be conveyed. 
The nature of the functions of the Vascular System has 
been the source of much difference of opinion. Spiral vessels 
have been supposed to be destined for the transmission of air, 
and it seems almost difficult to believe that a vegetable anatomist 
can doubt the fact; nevertheless Dutrochet and others assert 
that they serve for the transmission of fluids. Bischoff, who has 
instituted many ingenious and delicate experiments, with a view 
to the determination of this point, has arrived at the conclusion, 
“ that plants, like all other living bodies, require for the support 
of their vital functions a free communication with air, and that 
it is oxygen most especially which, when absorbed from the 
soil by the roots, renders the crude fluid fit for the support and 
nourishment of a plant, just as blood is rendered fit for that of 
animals ; but for this purpose, it is not sufficient that the ex¬ 
ternal surface should be surrounded by the atmosphere ; aerife- 
rous channels are provided in the form of spiral vessels, placed 
internally, and which convey air containing an unusual propor¬ 
tion of oxygen, obtained through the root from the earth and 
water by its own vital force/’ “ With such evidence,” says 
Dr. Lindley, “ of the aeriferous functions of the spiral vessels, 
