THE STEM. 27 



the value of many of the garden vegetables, which, in their 

 primitive, uncultivated state, are as unfit for food as any other 

 kind of root; but the effect of cultivation in good soil, is to 

 increase the cellular integument without increasing the liber ; 

 thus fitting the roots for becoming food for men and animals. 



49. In a great number of trees, the bark is make the deposi- 

 tory of important articles. Tannin, by which raw hides are con- 

 verted into leather, is found in the bark of several species of 

 the Quercus, or Oak, and in the Pinus Canadensis, or Hem- 

 lock. Gum is also an abundant product of the bark, and is 

 scarcely found in the wood. Coloring /natter is often found 

 deposited in this organ, though not so generally as in the wood : 

 also medicinal substances of every grade, from the mildest muci- 

 lage of the Ulmus fulva and Bene plant, to the most powerful 



ns of the Daphne mezerium. and of those yielding the 

 Hydrocyanic or Prussic Acid. Xumerous examples might be 

 added of the important productions of this organ, but they 

 will be reserved for a future section. 



50. The functions of the bark, taken as a whole, seem to be 

 the protection of the newly formed wood, the secreting of vari- 

 ous products, and forming a channel through which the descend- 

 ing elaborated sap may pass to the various parts of the stem. 

 The functions of the cellular integument and liber, separately 

 considered, have not been determined. There appears to be 

 plausibility in the conjecture, that the cellular part of the bark, 

 being deposited first, acts the same part in the formation of the 

 liber that the pith performs in a newly formed branch, that of 

 affording nourishment, if it does not act some part in generating 

 the fibers themselves. If, as has been supposed, the cellular 

 system is the generating apparatus of vegetables — and that it 

 is in some cases, we have the best evidence — will it not afford 

 a probable reason for the arrangement of the cellular and vas- 

 cular tissues in alternate layers, if we suppose that the cellular 

 tissue, being first deposited, acts as the generator of the fibrous 

 tissue of the liber ? 



Section 3. — The Stem. 



51. The stem is that part of the plant to which the leaves 

 and flowers are attached. There are three distinct varieties of 

 this organ, characterized by their manner of growth. 



The first is called dicotyledonous, from the fact that the seed 



49. What deposition made in the bark ?— 50. What are the functions of 

 the bark ? — 51. What is the stem ? How many varieties ? 



