THE STEM < 459 
autumn wood of the previous year, the rings can be seen with 
the naked eye, and the age of the tree detected. Secondary 
wood and bast must be distinguished from primary, as their 
origin is different. The wood and bast formed from the 
meristematic tissue at the growing point is known as primary ; 
that formed from the activity of the cambium, as just de- 
seribed, is secondary. | 
Tn a young stem the bundles are isolated, but as develop- 
ment proceeds a layer of cambium arises between the bundles ; 
it becomes united with the original cambium of the bundle, 
so that a complete cambium ring is formed. The cambium 
between the bundles is distinguished as inter fascicular. This, 
too, gives rise to wood and bast, so that if a section of a stem 
1s made in which this process is going on, young bundles will 
be seen between the first-formed bundles, which after a time 
grow as large as the original bundles ; ultimately the bundles 
become united, forming a complete ring, as in Dicotyledons. 
Comparison in Mono- 
pee cotyledons 
and Di. the bundles 
cotyledons. do not form 
4 complete ring; the 
stems of these plants 
do not increase in 
thickness, so that the 
cylindrical arrangement 
of the bundles found in 
Dicotyledons is not so 
hecessary in their case. 
Monocotyledons _—_are 
lower-growing plants, Fic. 27. —MoNOCOTYLEDONOUS STEM, 
TRANSVERSE SeEcrIonN. (Low power, 
and do not therefore 
diagrammatic. ) 
need to offer so much e, epidermis ; f.v.t, fibro-vascular tissue ; 
resistance to the force p, ground tissue of parenchyma. 

of the wind as many 
Dicotyledons do; the bundles are scattered, not in a ring. 
As the stems do not increase in thickness, there is no neces- 
j—2 
