THE STEM OF PLANTS. 33 
of the winds. Nature has constructed them with the particular 
aim in view of resistance. Year after year, she accumulates in 
their interior successive layers of increasingly solid substance. In 
proportion as the vegetable increases in size and needs a more 
powerful support, the interior concentric rings, which by their com- 
binations form the strong and compact tissues of our forest trees, 
are compressed, and more and more consolidated. In its origin 
—that is to say, at the moment when the young stem, just sprung 
Fig. 37.—Section of a Crocus Bulb. Fig. 38.—Bulb of the Crocus. 
up out of the ground, begins to rear itself in the air—nothing is 
observable in its interior except an abundance of pith, surrounded 
by its breathing vessels (trachee). But as the plant increases, 
new elements interpose between the pith and the bark; and when 
the trunk has lengthened and strengthened, it presents an internal 
structure complicated enough, and well calculated for resistance 
to all outward forces. A mere glance at the section of a log of 
tire-wood, informs us that the stems of forest trees present three 
D . 
