52. SOUTH AFRICAN BOTANY 
cencentric ring of wood indicates a year of the tree’s 
age. Thus the age of a felled tree can be ascertained 
by counting the number of rings of wood—ANNUAL 
Rineas—as they are called. The reason why these rings 
can be so clearly distinguished the one from the other is 
that the secondary wood laid down in the spring consists 
of large vessels, whilst that laid down in autumn consists 
of small vessels. If this were not the case the wood 
Fid. 34.—Transverse Section of a Five-year-old Oak Stem, 
m.r. Medullary ray. p. Pith. a, to, Rings of xylem formed during 
successive years. (From Darwin’s ‘‘Hlements of Botany ”’.) 
would be homogeneous in nature, and no line of separa- 
tion would be noticeable between one year’s growth and 
the next. Besides the secondary xylem and phloem 
there are present in an old oak stem rays of tissue run- 
ning radially outwards. Some—the primary medullary 
rays between the separate vascular bundles—were present 
in the seedling, the others have been produced later 
along with the rest of the secondary tissue by the 
cambium., 
