130 Field, Forest, and Wayside Flowers 
as it were, for new and higher uses. Each cell in 
such a cluster divides lengthwise into two, which 
again divide, each into two. This young tissue in- 
stinct with formative life is ‘‘procambium.”’ After 
a little while the cells on one side of it lengthen, 
their walls grow thicker, and on them appear the 
annular and spiral markings characteristic of the 
first-formed wood-vessels. On the other side of the 
procambium, meantime, bast-tubes are taking shape 
and office. 
In the palmetto-trunk, in the corn-stalk, and in 
the stems of most lilies the cells of the procam- 
bium soon cease to multiply, and they all become 
altered over into wood or bast before the close of 
the growing season. 
Thus Nature comes to the end of her material, 
and the growth of the bundle ceases perforce. 
Fibro-vascular bundles of this nature, which can 
grow ‘‘just so much and no more,” are called 
‘‘closed,” and are very general among monoco- 
tyledons. They are shut off from one another by 
masses of pith, and there is not, at any season, a 
continuous ring of young tissue running around 
the stem. So it is only in a few exceptional cases 
that the monocotyledonous stem grows. steadily 
thicker with age. 
