ANATOMY AND PHYSIOLOGY OF PLANTS. 221 



part to another, or to retain the proper juices secreted 

 by the plant. 



The vascular portion of the vegetable structure composes a kind 

 of net-work, owing to the free communication or asastomosis of tlie 

 vessels with one another, which pervades almost every part of the 

 plant. The particular vessels vary both in form and in the diameter 

 of their calibers. They are composed of the membranous texture, 

 are firm, comparatively thick, and somewhat pellucid. 



11. The vessels appear to constitute the bulk of all 

 plants, and under an idea that some conveyed sap and 

 others air, the phytologists of former days, called the 

 former sap vessels, and the latter trachea; or air tu!>es. 

 This opinion is now entirely rejected, and many beauti- 

 ful experiments have yielded ample evidence to sub- 

 stantiate the division into common vessels and proper 

 vessels. 



The former are stated to convey the sap or nutrient fluids from one 

 organ to another; and the latter to retain the proper juices formed 

 from the sap by the vital powers of the plants. 



12. The common or sap-vessels are most numerous, 

 usually occupying the chief substance of the internal 

 parts, and generally found to be of a spiral form. 



In a young branch of a tree or shrub, or in the stem of an her- 

 baceous plant, are found, ranged round the centre or pith, a number 

 of longitudinal tubes or vessels, of a much more firm texture than 

 the adjacent part, and when examined niinutelv, they oflcn 

 appear to have a spiral character. They are in fact, the common 

 vessels. They may also be discovered in the young twigs and leaf- 

 stalks of the elder, syringa, and many other shrubs, ns well as in 

 numerous herbaceous plants, as the peony, and more especially 

 many of the lily tribe. If a branch or stalk of any of theye plants be 

 partially cut through orgently broken, andits divided portions slowly 

 V '2 



