118 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 
leaves, in the shape of gum, resin, oil, etc., they are extracted 
and used for medicine and other purposes. In countries having 
no regular summer and winter, concentric rings are seldom visible 
in the wood of trees; but in Europe generally each concentric 
layer may be taken as the growth of one season, although it is 
not an infallible rule. 
Plates 11 to 21 are exceedingly good examples of the healthy 
tissue of Exogens, and they show the difference in the bark and 
the wood of the different types they represent; the greatest 
difference being found in the bark, which is well shown in a 
transverse section of the Abies excelsa on Plate 21. Bark may 
be said to perform the functions of clothing—protecting the 
young and tender woody formation from cold, drought, and 
other injurious influences,—but its principal work is to filter 
the elaborated juices as they pass from the leaves into the stem, 
which they do in a horizontal, or slightly declining direction. 
The residue of these juices passes into the roots. Hence the 
reason why so many secretions are found in the bark and roots 
compared with any other part of the tree. The elaborated sap on 
its descent passes horizontally through the pitted cells, or the 
cells with marginal borders. Examples of these cells are seen 
in the sections of the genus Pinus, and in those of Wellingtonia 
gigantea, Thuja orientalis, and Araucaria eacelsa, as well as in the 
dotted cells of the Mulberry. In Plate 22, a section of a young 
spruce, we have a very good example of the pitted cells with 
marginal borders. In Plate 24 is seen the double-rowed cells 
of the Wellingtonia gigantea; and also in Plate 26, the double 
rows of the Araucaria excelsa ; while on Plate 27, a section of 
the Mulberry shows quite a different form of these cells to that 
of the Pine tribe. 
When the concentric layers of woody tissue pass from the 
alburnum stage to that of the duramen, these pitted or dotted 
cells, vessels, and pith become clogged and cease to perform 
their functions. This is very apparent in the duramen of the 
Pine and other timber trees, in which, after a lapse of time, the 
secretions begin to dissolve, and are carried away into the soil. 
At this stage what is known as “heart-rot” begins. This rot 
affects the spiral tissue in the medullary rays, which in turn 
affects the health of the plant. If Plates 28 to 37 be compared 
with 38 to 44, the great difference in the structure of Pine 
timber when compared with that of the broad-leaved trees, is 
