196 Structure of Vegetables. 
the pith of a whole branch would be too little in some 
eases to supply one hour’s perspiration of a single leaf. 
Vessels of Plants .— -Plants are furnished with differ- 
ent kinds of vessels, which are distinguished from each 
other by their course, situation, and uses. The lym- 
phatic vessels, which serve for the circulation of the 
sap, are chiefly observed in the woody part of the 
plant. The peculiar vessels which contain thick or 
coloured fluids lie immediately under the bark. Some 
of these proper vessels are placed between the cuticle 
and the bark ; and some forming oval rings, and filled 
with the peculiar juices of the plant, are situated in the 
interior part of the bark. 
Another set of proper vessels is distributed in the 
alburnum, nearer to the centre of the trunk, and some- 
times in the perfect wood. 
The utrlculi, or cells, constitute another set of ves- 
sels, vary in form, colour, and magnitude in different 
vegetables, and exist in the roots, the bark, leaves, and 
flowers. They seem to resemble a flexible tube slight- 
ly interrupted with ligatures nearly at equal distances, 
while a free communication is preserved through its 
whole length. The tracheae or spiral vessels appear in 
the form of fine threads, and may be drawn out to a 
considerable length without breaking. These vessels 
are very numerous in all plants, form a kind of ring 
underneath the bark, and are distributed in distinct 
bundles in trees, shrubs, and herbaceous plants. These 
spiral vessels are easily detected in succulent plants, as 
in the leaf-stalks of Elder, Syringa, and other shrubs ; 
and in many plants of a herbaceous nature, as in the- 
Fseony and many of the lily tribe. 
