PHYSIOLOGY AND STRUCTURE OF TREES AND SHRUBS, 121 
All those trees that freely produce adventitious buds on their 
stems, such as the Alder, Elm, Elder, Lime, Oak, Poplar, Willow, 
and other trees and shrubs, are also those most capable of 
producing adventitious buds on their roots; the only conditions 
that are required being air, heat, and moisture, From an adven- 
titious, or normal leaf-bud, roots may spring, and instead of a 
branch we have an adventitious root on the stem, such as that of 
the Ivy shown on Plate 75. Foresters will have often observed 
such adventitious roots springing from almost all the hardwood 
or broad-leaved trees in clefts of the bole, and more especially if 
the heart of the tree is decayed. Adventitious buds are found 
in the medullary system of all trees, but owing to the persistence 
of the cortex they are unable to develop into leaves. The beech 
“knots” are good examples of this, as are also the excrescences 
found upon the Elm, Lime, and Sycamore, and sometimes upon 
Conifers, especially the Cedar. Owing, however, to the resinous 
properties of the sap of most Conifers, and the dry corky nature 
of their bark, adventitious leaf-buds are very seldom found upon 
them. When they are present they are most frequently seen 
upon Cedars, Firs, and Spruces, when these are growing in a 
moist climate. 
Leaves are simply a continuation of the bark, and have the 
same forms of tissue, which is fully illustrated on Plates 76 to 80. 
There is also a very good example in the leaf of the Zucalyptus 
globulus on Plate 83. The leaves are connected with the medullary 
sheath by spiral tissue passing up through the petiole or leaf 
stalk, An example of this spiral tissue is that from the Vine, 
shown on Plate 81. It then passes out into the blade or lamina. 
The latter is traversed by veins, which consist of bundles of the 
same vascular tissue as the petiole, and the whole is covered over 
with an epidermis peculiar to the species to which the leaf belongs. 
The epidermis of the leaf is perforated with openings called 
stomata, shown on the leaf of Boxwood on Plate 84. These 
occur upon both sides of the leaves of many plants, but generally 
upon the under side only, and they are very numerous, ranging 
from 12,000 to 200,000 in a square inch. The respiratory functions 
performed by these peculiar openings are well known. The 
remaining Plates, from 85 to 100, illustrate the interior structures 
of various interesting classes of plants in a similar manner to 
those already described. 
