Add one--R. F. Evert
One of Evert's main problems was to determine when and where the first
food-conducting cells are produced when growth is resumed in the spring.   He found
that the precursors of these cells are already present during the winter in the
cambial region. The cambium is a perpetually young tissue situated between the
phloem and the xylem, and responsible for the production of new phloem and xylem
each year.
"Early in April, before the buds begin to break and new leaves are
produced, a few rows of cambial cells enlarge to produce new food-conducting cells,"
Evert explains. "And, unlike the pattern of growth reported for other trees, the
cambial region continues to produce new phloem for about one and one-half months
before it begins new xylem production."
The scientist goes on to say that in spite of this late start, xylem
production is much greater than phloem production, and generally continues for a
longer period of time. Therefore, during any given year, much more wood is usually
produced than phloem.
He points out that practically all growth is completed by the middle
of August. Late in September, the first formed food-conducting cells begin to
die, and by late November or early December, all food-conducting cells are dead.
The cycle then repeats itself the following year.