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 v»-ith 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 



