lU 



PnrSIOLOGICAL VIEW8. 



more value " than the lilies of the field," or than " many sparrows V It is the very 

 principle within us which enables us to make this inquiry that renders us thus 

 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 

 tiie 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 nevj woody layers ex- 

 ternally ; consequently, the oldest and 

 densest part of the stem is near the cen- 

 ter. 



Duratio7i of the trunk indefinite ; it is 

 destroyed only by disease and external 

 caxises. 



Root conical, destitute of pith and 

 spiral vessels. 



Native, as trees in all climates. 



ENDOGENOUS PLANTS. 



Trunk usually simple and cylindrical. 



Rind smooth, not made 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 iyicreases in diameter by the 

 formation of new woody bundles within 

 the old wood; consequently, the oldest and. 

 densest portion of the stem is at the cir- 

 cumference. 



Duration 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. 



LECTUEE 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 vegetaljle anatomy in connectiomvith pJiysiology : that is, 

 when treating upon each particular orgai^, we have remarked 

 upon its functions in the life and groioth of the whole plant. 

 I et, although we have attempted to show how plants groio.^ it is 

 not easy to explain how they live. The great jprinciple wMch 

 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 the 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 wliy 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 and Endogenous plants, 

 oniy and physiology'. 



-139. Connection of vegetable anat- 



