CHAP. I. 
ELEMENTARY ORGANS. 
249 
answer such an end. It consists, as has been seen, of ex- 
tremely slender tubes, each of which is indeed possessed of 
but a slight degree of strength; but being of different lengths, 
tapering to each extremity, and overlapping each other 
in various degrees, these are consolidated into a mass that 
considerable force is insufficient to break. Any one, who will 
examine a single thread of the finest flax with a microscope 
that magnifies 180 times, will find, that that which to the eye 
appears a single thread, is in reality composed of a great num- 
ber of distinct tubes. 
It is also the tissue from which roots are emitted. Unlike 
the leaf-buds, roots are always prolongations of the woody tissue 
of the stem, as may be readily seen by tracing a young root 
to its origin. 
The real nature of the functions of the vascular system has 
been the subject of great difference of opinion. 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 that fact. Nevertheless, many others, and among them 
Dutrochet, assert that they serve for the transmission of fluids 
upwards from the roots. This observer states, that if the end 
of a branch be immersed in coloured fluid, it will ascend in 
both the spiral vessels and vasiform tissue ; 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 been, or perhaps has 
been, added the questions, why they do not exist in the wood, 
where a movement of sap chiefly takes place in exogenous 
trees ? and also, how it happens that their existence is almost 
constantly connected with the presence of sexes, if they are 
only sap-vessels ? And further, it has always been remarked, 
that if a tranverse section of a vine, for instance, or any other 
plant, be put under water, bubbles of air rise through the 
