LECT. III.l VASCULAR TEXTURE. 77 



aimed at by injecting coloured wax and mercury 

 into the animal vessels, when we wish to trace 

 their course. The most eligible fluids for this 

 purpose are decoctions of Brazil wood, and infu- 

 sions of the skins of black grapes ; and the plants 

 likely to yield the most satisfactory results to the 

 beginner, are the Periploca Grceca, and the Aris- 

 tolochia Sipho*. The plant or twig to be injected 

 should be cut with a very sharp knife, and its 

 divided end immediately placed in the coloured in- 

 fusion in a warm temperature : after a few hours the 

 colour, in plants favourable for the experiment, may 

 be traced into the leaves, the flowers, and even the 

 fruit. This discovers the course of the conducting 

 or adducent vessels ; and when the operation is 

 reversed, the twig being cut at its top, and in- 

 verted in the coloured fluid, we can trace that of 

 the returning or abducent vessels* By placing 

 transverse and longitudinal sections of twigs and 

 parts of herbaceous plants thus treated under the 

 microscope, we are able to ascertain the organiza- 

 tion of the coats of the vegetable vessels. Some 

 of the vessels, however, cannot be rendered more 

 visible by this means, as they refuse to admit 

 coloured fluids ; and therefore any knowledge of 



* This species of Aristoloohia is a native of the Alleghany 

 mountains ; but it is now to be found in every botanical garden, 

 and in most of the nursery gardens around London. 



