174 VASCULAR SYSTEM. [BOOK n. 



With regard to its giving firmness and elasticity to every part, 

 we need only consider its surprising tenacity, as evinced in 

 hemp, flax, and the like ; and its constantly surrounding and 

 protecting the ramifications of the vascular system, which has 

 no firmness or tenacity itself. To this evidence might be 

 added, the admirable manner in which it is contrived to 

 answer such an end. It consists, as has been seen, of lignified 

 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 which 

 considerable force is insufficient to break. Any one who will 

 examine a single thread of the finest flax, with a microscope 

 which magnifies only 180 times, will find that what to the 

 eye appears a single thread is in reality composed of a great 

 number 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 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 

 bothrenchym ; 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 



