156 PHYSIOLOGICAL VIEWS. 



LECTURE XIX. 



Chemical Composition of Plants. 



WE have, according to our method of arrangement, con- 

 sidered the anatomy of the vegetable in connexion with its phy- 

 siology : that is, when treating upon each particular organ, we 

 have remarked upon its uses in the life and growth of the whole 

 plant. We have treated of the germination of the seed, the 

 minute vessels which constitute the vegetable fabric, with tho 

 fluids which circulate through these vessels ; we have, as it 

 were, gathered these vessels together and 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 monocotyle- 

 donous and dicotyledonous plants. 



Yet, although we have seen how plants grow, it is no easy 

 thing to explain how they live. The great principle which 

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

 the eye of man. But by a careful observation of facts, we can 

 learn all that it is important for us to know in order to culti- 

 vate plants successfully ; 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 observa- 

 tions, made by himself or others. 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 is able to give some rea- 

 sons, he is ultimately arrested in his speculations by a barrier, 

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

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

 tells you that these are alkalies, substances which neutralize 

 acids, and thus render them harmless ; inquire still farther, 

 why alkalies do thus affect acids, and the physician is as igno. 

 rant as yourselves. 



Before closing our view of the vegetable structure, we shall, 

 by the aid of chemistry, examine the elements which com- 

 pose it. 



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

 show that they imbibe some external substances, which are in- 

 Retrospective view of subjects considered Difficulty in investigating the 

 life of plants How far can a physician enlighten others ? Examination of the 

 elements which compose the vegetable structure. 



