Tree (Ginkgo biloba . LI). 131 
of the pith into the xylem-ring (Fig. 24, p, p) ; a third smaller 
wedge (Fig. 25, p') makes its appearance between the two 
larger ones, and this rapidly extends in a radial direction 
until the vascular cylinder is broken through (Fig. 26); finally, 
the two groups of xylem and phloem 1 become free (Fig. 27), 
and pass outwards (Fig. 53, It) into the leaf-stalk accompanied 
by one or more large secretory canals ( 5 ). Close to the 
base of the leaf-lamina each bundle divides into two and then 
breaks up into the dichotomously branched veins. 
ii. Young stem (seedling). 
A transverse section of a young stem 2*5 mm. in diameter 
presents the following features:—The epidermis iscuticularized, 
but no cork has been formed in the hypodermal tissues ; the 
xylem occurs as a ring made up of groups separated by wide 
medullary rays, each group being composed of radial rows of 
tracheids and medullary rays one to two cells in width; the 
phloem is approximately equal to the xylem in depth, and 
the crushed protophloem is succeeded by secretory cells. 
The protoxylem elements are often compressed and separated 
from the radially disposed tracheids by one or two layers of 
parenchyma. Long secretory sacs occur in the pith and 
canals traverse the cortex ; the cortical region also includes 
scattered cells containing crystals of calcium oxalate. 
iii. Short and long Shoots. 
The type of stem represented by a seedling is also met 
with in the terminal elongating shoots of the adult tree; the 
phyllotaxis of these long shoots may be §, f, or x % ; each leaf 
bears in its axil a bud which in the third year often develops 
into a short shoot producing a few crowded leaves. The 
short shoot elongates from year to year and bears an apical 
group of leaves, the older portion being covered with the 
crowded leaf-scars of former years, presenting an appearance 
1 Cf. Cordaites ( Dadoxylon ) as figured by Williamson (’77), PI. IX, pp. 44 
and 46 . 
K 2 
