36 THE VEGETABLE WORLD. 
‘ wood (duramen) to the innermost ones. In some trees, and 
notably in those which are not hard grained, as the poplar, 
willow, and chestnut, the line of demarcation between the wo 
and the sap-wood is slightly marked. In hard woods, on the 
contrary, it is strongly defined. Thus, in ebony the heart-wood 
is of an intense black, whilst the sap-wood is white ; in the Judas 
tree the heart is yellow and the sap-wood white ; in the Phillyrea 
(mock privet) the heart is red and the auder, or sap-wood, white. 
Workmen who work in wood are well aware that the sap-wood 
is much less solid than the heart, and that the latter only ought 
to be employed for wood work. Examined in masses, the igneous 
layers are hardest at the centre; but (Fig. 48, woody fibres 
__ magnified) studied individually, each layer is more com- 
iq || pact towards the exterior. Neither are all the layers of 
}|} an equal thickness, whether compared with each other, 
i44)95] or in their several parts. 
| The substance which prevails in the wood, and that 
| Iinating in a point at the two extremeties, as there re- 
| presented. The walls of the ‘cells of which it is composed 
cavity 1s much reduced. This thickness, as well as the 
entangled as to constitute what is called a fibrous tissue, 
|} very difficult to pierce when cut across, but, on the con- 
m| trary, easy enough to divide when cut longitudinally. 
This igneous fibre is not the only element composing 
which the substances we are going to speak of acquire a 
the branch is straight, you will see the light at the other 
which gives it its hardness, is the woody fibre, represented : 
|.in Fig. 48. This is an elongated cellular substance, ter- 
i} are very thick, generally so thick that their interior 4 
colouring of the fibres, varies with the different parts of : 
wood, with the age of the stem, and even with the nature — 
of the tree that is under examination. The woody fibres — 
be 
4 
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a 
4 
: 
: 
ti 
: 
f 
Z 
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press end to end one against the other, and become 80 : 
ee ee eae hee Sew NS ee 
JN} wood. Cut transversely a branch of the vine (a plant in 4 
considerable volume), and apply the eye to one end; if 3 
end. Examine the surface of the section of the branch, either — 
with the naked eye or with a magnifying glass, and it will be 
observed that it is perforated with a considerable number of small 4 
