132 Field, Forest, and Wayside Flowers 
dry and apparently half dead. Inside this is a 
layer of green bark, full of sap and vitality. 
Beneath this lies the wood—a hollow cylinder, 
enclosing a light porous substance—the pith. 
This marked division of the stem into concentric 
rings of bark, wood, and pith is found only in the 
dicotyledons. 
In a very young plant of the rose’s kin this dis- 
tinction is not yet apparent. 
Indeed, a cross-section of any very young flower- 
ing-plant shows a stem-tissue alike in every part. 
In the lily’s kin it all behaves alike, for any clus- 
ter of cells anywhere in the young stalk may turn 
into procambium. 
But when a young plant of the rose’s kin is 
about to acquire fibro-vascular bundles the little 
clusters of cells which become instinct with con- 
structive life lie just beneath the surface of the 
stem. 
Next spring’s bundles will develop in the 
spaces between those of last spring, and by time 
the stem is four or five years old it has a ring of 
bast running all around it and a ring of wood 
within. Between these, in spring, there is a circle 
of cells which are actively at work building up new 
tissue. 
