38 THE VEGETABLE WORLD. 
is marked with small dots, furrows, and streaks, they are called 
dotted vessels and striated vessels. 
There is a particular part of the wood, however, in which the vessels 
are of a very different nature from that just indicated. We find them 
round the pith, in the innermost portion of the woody circle, and 
never anywhere else. These vessels, with the slender fibres which 
accompany them, have received, and most improperly retained, 
the name of the medullary sheath. We say improperly, for here 
there is only a combination of vessels, and no sheath. The image 
which this word recalls is not of a nature to enable us to under- 
stand the important modification of structure which belongs to the 
innermost vessels of the woody circle, which 
e2 are the air-vessels, or trachee, of the system. 
Fig. 45 represents the central part of @ 
| piece of tree as seen in the microscope, with 
a very strong magnifying power. In this 
central portion, which has been most im- 
properly called the medullary sheath, we see — 
the trachea, that is to say the air-vessels, of | 
which we are speaking, on one side touch- 
ing the pith at the centre of the stem, 
on the other side in contact with the woody 
re. 
The structure of these tracheii is very sin- 
gular. They form masses of elongated fibre, 
which is still more slender at the extremities. 
Any one would believe at the first glance that 
these vessels were very finely streaked diago- 
nally, and that their external coat is con- 
tinuous ; but if the slightest pulling is used 
towards ‘thin, they unroll like a spiral spring. 
These vessels are, then, formed of a spiral, 
— thread, twisting round a cylinder with con-_ 
Fig. 45.—Trachee of plants 4° . A mene 
surrounded by pith and woody tiguous spiral turns, which are joined together 
by a membrane, which is so extremely thin, 
that it is difficult to find the traces of it when the spiral tube has _ 
been unrolled. 
There is one final peculiarity which marks the section of the | 
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