CHAP. II. 
ELEMENTARY ORGANS. 
299 
tapering to each extremity, and overlapping each other in 
various degrees, these are consolidated into a mass which 
considerable force is insufficient to break. Any one who 
will examine a single thread of the finest flax, with a micro- 
scope which magnifies only 180 times, will find that what to 
the eye appears a single thread is in reality composed of a 
"reat number of distinct tubes. 
O 
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 seen by tracing a young root to 
its origin. The woody tissue, when applied to this purpose, 
is, however, always covered with cellular tissue. 
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 observers, and among 
them Dutrochet, assert that they serve for the transmission of 
fluids upwards from the roots. This physiologist states that, 
if the end of a branch be immersed in coloured fluid, the latter 
will ascend in both the spiral vessels and articulated bothren- 
chyma ; 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. 
But from this statement it does not appear that spiral vessels 
convey fluid ; it only shows that M. Dutrochet confounds one 
kind of tissue with another, when he infers that trachenchyma 
performs certain functions, because such functions are proper 
to bothrenchyma in a particular state. It has also been asked, 
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 question, why they 
do not exist in the wood, where a movement of sap chiefly 
takes place in exogenous trees. 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 water from the mouths of the spiral 
