FOOD. 87 



230. Many theories have been advanced to account for 

 the motion of the sap, all of about equal consideration. 

 Some attribute it to evcqjoration through tlie leaves, others 

 to cajjillary atiraction, others to vital action, I confess that 

 no theory that I have ever seen satisfactory explains this 

 circulation. All are plausible up to a certain point, when 

 some stubbon fact intervenes and checks one's pride of opin- 

 ion as effectually as a frosty night does the flow of sap. 

 To the one mysterious power, vital action, must the phe- 

 nomenon be ascribed. 



231. You have now been told of the sap and its offices. 

 Let us take a hasty glimpse at some of the substances 

 elaborated by it. The first in importance is starch. How 

 important can be imagined when I tell you that more than 

 one-half of the human family live entirely upon it in its 

 various forms. Here we also find sugar, gum, oil, Avax, 

 resin, mucilage, alkaloids, narcotine, morphine, cinchona, 

 iodine, potash, soda, and poisons, both narcotic and acrid. 



232. The sap elaborates all these, but certain cells are 

 formed which secrete from it some of these substances ; 

 others seem to be thrown out from the sap as excessive ; 

 and hence we divide them into two classes, calling tlie first 

 secretions, and the latter excretions. Sweet-gum (Liquid- 

 amher, styracifiua) is an excretion of the sweet-gum tree, 

 while tannin in the red-oak [Quercus rula) bark is a 

 secretion. 



233. We have now in discursive chapters learned of vari- 

 ous vegetable phenomena : such as the irritability of plants ; 

 their color; their permanent, fugitive, and intermittent 

 odors ; the influence of light upon tissue ; thcproperties and 

 longevity of plants; of their food, organic and inorganic, 



230* Has any satisfactory theory been advanced to account for the upward flow 

 of sap ? To what must we ascribe it ? 



231. Tell some of the substances eliminated by the sap. "Where are these 

 found ? 



232. What are the two kinds of cells called? Sweet-gum? Tannin? 



233. What have you now learned ? 



