I 2 I 
Tree (Ginkgo biloba, L.). 
of irregular outline, those of the upper surface being slightly 
larger than those of the lower; the cuticle is well marked but 
not very thick, and the stomata, with their guard-cells some¬ 
what below the level of the epidermis, are confined to the 
lower face of the lamina. Bertrand 1 states that there is no 
palisade tissue in the mesophyll; this is true of the smaller 
leaves on the flowering shoots, but the cells next the upper 
epidermis of the larger leaves are distinctly elongated at right 
angles to the surface. These palisade elements are rather 
irregular in form and often lobed, as are also the smaller 
mesophyll cells; the latter are usually elongated parallel to 
the leaf surface, and separated from one another by numerous 
intercellular spaces. Short canals occur between the veins, 
and a group of secretory cells is found above and below each 
vascular bundle, which may be in contact with the phloem, 
but are often separated from the xylem by one or more 
layers of parenchyma. The xylem is formed of about a 
dozen tracheids, which may occur either as one group or as 
several smaller strands separated by medullary rays ; the 
latter appears to be the more usual arrangement in leaves 
from a fruiting branch, and the former in the ordinary foliage 
leaves. As Worsdell 2 points out, the centripetal xylem is 
considerably reduced, and is represented by one or two 
tracheids. On either side of a bundle there are often one 
or two transfusion-elements having the form of reticulate 
tracheids of larger diameter than the ordinary xylem-tracheids. 
Some of the medullary-ray cells may be enlarged and serve 
as secretory elements ; and a few parenchymatous cells con¬ 
taining crystals are present in the neighbourhood of each 
vascular bundle. There is no sclerenchymatous tissue, and no 
thick-walled hypoderm. 
The petiole is traversed by two vascular bundles inclined 
towards one another, the xylem being made up of regular 
rows of pitted tracheids, numerous medullary rays, and a few 
centripetal elements ; the transfusion-tracheids vary in number 
1 Bertrand (’ 74 ), p. 30. 
2 Worsdell (’ 97 ), p. 306; vide also Zimmermann (’ 80 ), p. 5. 
