CHEMICAL COMPOSITION OP PLANTS. 119 



within you wiiich enables you to make this inquiry, that renders 

 you thus precious ; — it is your soul that raises you above the inani- 

 mate and brute creation. Though the body is sister to the worm 

 and weed, the soul may aspire to the fellowship of angels. Oh, then, 

 let me entreat you, suffer not your chief thought to be given to the 

 decoration of the perishable part, the mere temporary dwelling-place 

 of the immortal mind ! but seek to prepare this mind for admission 

 tnto " the glorious company of the spirits of the just made perfect." 



LECTURE XX. 



PHYSrOLOGICAL VIEWS — CHEMICAL COMPOSITION OF PLANTS — PROXIMATE PRIN- 

 CIPLES — CHEMICAL ANALYSIS OF THE SAP, 



We have, according to our method of arrangement, considered 

 the anatomy of the vegetable in connexion with its physiology : that 

 is, when treating upon each particular organ, we have remarked 

 upon its uses in "the life and groicth of the whole plant. We have 

 treated of the germination of the seed, the minute vessels which con- 

 stitute the vegetable fabric, with the fiuids which circulate through 

 these vessels ; we have considered them as constituting, in various 

 ways, three essential parts of woody plants, the bark, wood, and pith. 

 We have inquired into the manner in which these separate parts 

 are formed, and observed the great distinction in the growth of the 

 stems of monocotyledonous and dicotyledonous plants. 



Yet, although we have attempted to show how plants grow, it is 

 no easy thing to explain hoAV they live. The great principle which 

 operates in organic life, appears not to have been laid open to the 

 eye of man. Bat by a careful observation of facts, we can learn all 

 that it is important for us to know, in order to cultivate plants suc- 

 cessfully ; their habits, food, and the causes of their diseases and 

 death. 



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 reUef of a particular dis- 

 ease ; but he knows not why this should be so ; or if he is able to 

 give some reasons, he is ultimately arrested in his speculations by a 

 barrier which he cannot pass. Thus he knows that soda or pearl- 

 ash corrects acidity in the stomach ; ask the reason of this, and he 

 tells you that these are alkahes, substances which neutralize acids, 

 and thus render them harmless ; inquire still further, why alkalies do 

 thus affect acids, and the physician is as ignorant as yourselves. 



_ Before closing our view of the vegetable structure, we will, by tho 

 aid of chemistry, examine the elements which compose it. 



The growth of vegetables, and the increase of their weight, show 

 that they imbibe some external substances, which are incorporated 

 into their own substance. This constitutes nutrition, and distin- 

 guishes living substances from dead matter. A stone does not re- 

 ceive nourishment, although it may increase by an external accumu- 

 lation of matter. "Vegetable substances, analyzed by a chemical 

 process, have been found to contain carbon, oxygen, hydrogen, and 

 sometimes nitrogen, sulphur, silex, the oxide of iron, soda, magnesia. 

 and chalk."* These different substances are by the root, stems, and 

 leaves of the plant, derived from the earth, air, and water. 



* Mirbel, " Elemen s de Botanique." 



Recapitulation— A difTerence between the knowledge of facts, and of their causes* 

 Substances which compose plants. 



