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. 



