114 
PHYSIOLOGICAL VIEWS. 
more value " tlian the lilies of the field," or than " many sparrows ?" It is the very 
principle wJtliin us which enables us to make this inquiry that renders us thua 
precious ;- -it is the soul that raises us above the inanimate and brute creation ; 
and though the body is sister to the worm and weed, the soul may aspire to the 
fellowship of angels. 
138. The principal differences between Exogenous and En- 
dogenous plants, so far as they depend upon the structure of 
the stem, may be exhibited as follows : 
EXOGENOUS PLANTS. 
Trunk conical, generally branched. 
Bark separable from the stem, made 
up of annual layers ; in old trunks, 
cracked or broken into fissures externally. 
Wood composed of concentric zones, 
traversed by medullary rays, with a dis- 
tinct central pith. 
Stem increases in diameter by the suc- 
cessive addition of new woody layers ex- 
ternally ; consequently, the oldest and 
densest part of the stem is near the cen- 
ter. 
Duration of the trunk indefinite ; it is 
destroyed only by disease aiid external 
causes. 
Root conical, destitute of pith and 
spiral vessels. 
Native, as trees in all climates. 
endogenous plants. 
Trunk usually simple and cylindrical. 
Rind smooth, not mode up of con- 
centric layers; inseparable from the 
stem. 
Wood consists of fibrous and vascular 
bundles, not arranged in concentric lay- 
ers; no distinct pith, no medullary rays. 
Stem increases in diameter by tha 
formation of new woody bundles within 
the old wood ; consequently, the oldest and 
densest portion of the stem is at the cir- 
cumference. 
Jburation and increase of the trunk 
confined within certain limits; life ter- 
minable from the solidification of its 
tissue. 
Root cylindrical, of the same structure 
as the stem ; sometimes furnished with 
spiral vessels. 
Native, as trees only in hot climates. 
LECTURE XXI. 
PHYSIOLOGICAL VIEWS. CHEMICAL COMPOSITION OF PLANTS. PROXI- 
MATE PRINCIPLES. CHEMICAL ANALYSIS OF THE SAP. 
139. We have, according to our method of arrangement, con- 
sidered vegetable anatomy in connection with jpJiysiology : that is, 
when treating upon each particular organ, we have remarked 
upon its functions in the life and growth of the whole plant. 
1 et, although we have attempted to show how plants grow^ it is 
not easy to explain how they live. The great principle which 
operates in organic life is not laid open to the eye of man 
The physician who spends a long and laborious life in the study 
of tlie human frame, can give only the result of his observation. 
He finds a certain article efficacious in the relief of a particular 
disease, but he knows not why this should be so ; or if he be 
able to give some reasons, he is ultimately arrested in his spec- 
ulations by a barrier which he cannot pass. 
138. Differences between Exogenous an'l Eadogenous plants.— 139. Connection of vegetable anat 
jmv and physmlogy. 
