VESICULAR VESSELS.—EPIDERMAL TISSUE. o7 
(fig. 119) ; or it may be mucilaginous, or gummy, or contain 
active secretions, &c. Laticiferous vessels, from their containing 
some of the so-called secretions of the plant, are closely allied 
to the Receptacles of Secretion, and are frequently placed with 
them. (See page 72.) Laticiferous vessels occur especially 
in the inner bark of many Dicotyledons, in the pith, and in the 
petiole and veins of leaves; but they are also to be found in 
other plants. ; 
They are formed, like other vessels, from rows of cells 
arranged in various directions with respect to one another, the 
yartitions between their cavities being more or less absorbed 
so that they communicate freely together. 
Besides the above more common characteristics of laticiferous 
vessels, there are numerous other varieties ; indeed, from the 
very great variety in structure, contents, and position of these 
vessels, and the many and various transitions between them and 
vesicular vessels, now to be described, Sachs has proposed that 
these laticiferous and vesicular vessels should be included under 
the common name of latex-sacs. 
h. Vesicular or Utricular Vessels.—These resemble laticiferous 
vessels in one particular, as they contain latex (which, however, 
is clear or milky, but always contains true raphides) : while, on 
the other hand, they are unbranched and analogous to sieve- 
tubes in form, consisting as they do of long broad cells with 
sieve-like septa. They were first noticed by Hanstein in the 
scales of the bulb of Alliwm, and have since been observed in 
the leaves and other parts of Monocotyledons, and in some 
Dicotyledons. 
We have now described all the different kinds of cells, and 
the modifications they undergo, and the combinations of them 
which take place so as to form vessels. The different kinds of 
vessels and woody tissues are more or less combined together, 
and have always a tendency to develop and arrange them- 
selves in longitudinal or vertical bundles in the parts of the 
plant where they are found, and thus they may be readily dis- 
tinguished from the parenchyma in which they are placed, both 
in their form and mode of elongation. We thus find it con- 
venient to speak of the tissue formed of these bundles under 
the collective name of Fibro-vascular Tissue, or the Fibro-vascu- 
lar, Vertical, or Longitudinal System, to distinguish it from the 
ordinary cellular tissue, which constitutes the Parenchymatous, 
Cellular, or Horizontal System. 
4, Eprermat Tissv—E.—In Cormophytes and in all Flower- 
ing plants, the cells situated on the surface of their young 
parts and organs (see page 60) which are exposed to the air, vary 
in form and in the nature of their contents from those placed 
beneath them, and are so arranged as to constitute a firm layer 
which may commonly be readily separated as a distinct mem- 
brane. To this layer the term Epidermal Tissue is given. It 
