50 MR. YOU ATT S LECTURES 
The texture is fibrous as in the animal, and the fibres run 
longitudinally, and are crossed by others, so as to form a net-work 
of vessels—for the fibres of plants, whatever may be said of those 
of animals, are evidently vascular. (Several drawings were here 
exhibited.) In these tubes the sap and juices are conveyed and 
elaborated. 
The proportion of the solids to the fluids is far greater in plants 
than animals; therefore a tree retains its bulk for centuries, while in 
a short period nothing is left of the animal but a few bones. In 
the animal the solids are supposed to occupy but one-sixth of the 
bulk—in the plant three-fourths; but this must be subject to much 
variation. 
The trunk is divided into the cortex or cuticle (the outer bark); 
the liber or cutis (the inner bark); the alburnum, or soft wood; the 
lignum, or hard wood, and the pith or medulla. This was illus¬ 
trated by engravings. The trunk enlarges, by the annual con¬ 
version of a portion of the cortex into liber—of the whole of the 
liber into alburnum, and the alburnum into lignum. Therefore 
the age of a branch, or even of the trunk, may be calculated by 
the number of concentric rings. 
The pith is a reservoir of nourishment and moisture in young 
branches, and without which the leaves would sometimes perish 
by drought. The pith is gradually obliterated in the larger 
branches. 
The leaves constitute the clothing, and much of the beauty of 
the plant; but they are mostly important as the organs of respira¬ 
tion. The upper side of the leaf is usually smooth, the under is 
ribbed; and these ribs consist of numerous canals, communicating 
with and running into the midrib, which is a continuation of the 
stalk. (The skeleton of a large leaf was here produced). Between 
these vessels are numerous others, branching and anastomosing in 
every direction, forming a plexus, or net-work of vessels, and the 
meshes filled with a cellular parenchyma. The upper surface of 
the leaf is constantly exposed to the sun. If it be turned, it will 
speedily resume its former position. 
The sap is conveyed through the foot-stalk, and passes through 
numerous small vessels beneath the cuticle of the upper suiface. 
It is thus exposed to the influence of the atmosphere, and to 
a variable quantity of light and heat. It is then returned through 
the vessels on the under side, and passes through the under por¬ 
tion of the foot-stalk. It ascends through the trunk to the upper 
surface of the leaves, and descends from the lower surface through 
the bark. 
^ -The sap, passing through the parenchymatous substance ol the 
leaves, undergoes a very considerable and important change, and 
