'20 THE STEM. 



growing in this country, which were large trees before mo 

 time of Columbus. Indeed it is probable that many trees 

 li\ c at least a thousand years, and it is by no means certain 

 that there are not trees now living which were planted before 

 the Christian era. On the contrary, some trees which attain 

 to a considerable size live only for a short period. In this 

 country the Lombardy Poplar, an exotic, begins to decay 

 soon after it arrives at maturity, and sometimes perhaps even 

 sooner. In general the longevity of a tree, like that of an 

 animal, is in proportion to the time required for it to come to 

 maturity, or attain its full growth. Thus the Oak grows for 

 centuries, while the Lombardy Poplar attains its full size in 

 twelve or fifteen years. 



Medullary or Pithy Stem, (Caulis medulosus.) The me 

 dulla, or pith, is the well known soft, compressible, light sub- 

 stance contained in the centre of many ligneous plants, as 

 the Elder, (Sambucus.) Many plants when young contain 

 pith, which is replaced by wood as they grow old. The 

 young shoots of common Elder always contain the largest 

 pith, though their stems may be only half the size of those of 

 the parent. Hence in this plant, it must be inferred, either 

 that the pith is converted into wood, or that the woody fibres 

 increase around it. 



It seems to be a provision of nature, that where the stem 

 is medullary, the wood surrounding it should be peculiarly 

 strong and compact. A species of Palm, which attains to a 

 considerable height, is only three inches in diameter, and 

 nearly two and a half inches of this is composed entirely of 

 pith. But to compensate for this want of strength in the in- 

 terior, the ligneous, or external part, is so hard and elastic as 

 rather to resemble whale-bone than the wood of other trees. 

 The cane fishing rod is another example of this provision. 



The pith of some trees is of considerable consequence as 

 an article of food. The Sago of commerce is made of the 

 pith of a species of Palm, (Sagus Rumphii,) single trees of 

 which are said sometimes to yield six hundred pounds of this 

 nutritious substance. 



Hollow, or Tubular Stem, (Caulis tubulosus.) Hollow 

 stems are not uncommon in garden plants. Fennel, Dill, 



What is said of the longevity of trees ? Is there any proportion be- 

 tween the longevity of trees and the time of their full growth ? What is 

 meant by a medullary stem ? In common elder do the young or old shoots 

 contain the largest pith? What is said of the strength of medullary 

 Btems ? Is any use made of the pith of trees ? 



