THE STEM OF PLANTS. 37 
holes, of unequal size. If you introduce a hair or a very fine 
thread into one of these openings, you will succeed in passing the 
thread to the other end of the branch. Continuous canals, there- 
fore, exist in the interior of the vine branch. These canals, 
formed of a membrane peculiar to them, are the sap vessels. 
If a portion of the transverse surface of a log of oak or elm is 
neatly cut, it will be observed that the inside edge of each ligneous 
zone presents a certain number of small holes, clearly perceptible 
to the naked eye, or at least with the help of a common lens: 
these are the orifices of rather voluminous vessels. In the centre 
of the ligneous zone the vessels are much smaller, and sometimes 
almost imperceptible. Examine the woed of the hornbeam, lime, 
or maple-tree, in the same manner, and it will be observed that the 
internal edge of the zone is not now occu- 
pied by large vessels, but is almost entirely 
riddled by the orifices of smaller and more 
evenly-sized vessels, which become indis- 
tinct towards ‘the external edge of each 
zone. These are called the /ymphatic 
What is the structure and function of 
these different vessels? They resemble a 
cylinder, with obstructions placed at inter- 
vals, more or less marked, hoving also trans- 
verse lines or diaph ti g them 
at brief distances, Sometimes the remains 
of these diaphragms correspond in the in- 
terior of the vessel to these contractions and 
lines. In one word, these cylinders appear 
formed of cells, placed end to end, the par- 
titions of which have been destroyed. Their 
exterior walls show certain points, rays, 
and net-work, presenting very beautiful | 
forms, which result from the unequal thick- “ 4s 
ness of the walls, inequalities which are PIB 3 Pryemrthie textes of 
the result of certain laws regulating their Dene 
existence. Fig. 44 represents these vessels in the melon. From 
the peculiar appearance offered by their external covering, which 
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