FORMS AND HFSTEMS OF TISSUES. 
87 
Papayacese {Carica and Vasconcella) the laticiferous vessels, on the other hand, run 
through the xylem-portion of the fibro-vascular bundles; they — i.e. the cells by the 
coalescence of which they are formed— are repeatedly produced in layers from the 
cambium with the other elements of the xylem ; pitted and reticulately thickened 
vessels alternate with them. The branches of the laticiferous vessels envelope these 
in all directions, and are sometimes firmly fixed to them superficially; but hori- 
zontal branches of these tubes also penetrate the medullary rays, and terminate, 
towards the primary cortex, in scat- 
tered ramifications or recurrent knots, 
as also in the pith if the stem, is hollow. 
As in the other families, a copious 
anastomosis of laticiferous vessels is de- 
veloped in the horizontal partition-walls 
which the medullary tissue forms at the 
origin of each petiole in the hollovv^ of 
the stem, penetrating the horizontal 
partition-wall in countless ramifications 
and in several layers one over another, 
and connecting the vessels belonging to 
the medullary rays with these of the 
whole wood-cylinder. In Papaveracese 
{Chelidonium, Papa'ver, Sangumaria) the 
laticiferous vessels are also very per- 
fectly developed; they are not here, 
however, as in the families just named, 
united into ribbon-shaped groups, but 
run mostly at a greater distance from 
one another, dispersed through the 
phloem and the surrounding paren- 
chyma; single ones appear also in the 
pith, but do not penetrate into the 
xylem. Lateral outgrowths and cross- 
anastomoses are seldom found in the 
stem, but abundantly in the leaves, and 
especially in the carpels, in which close- 
meshed reticulations are formed, ac- 
cording to Unger, in the parenchym.a- 
tous fundamental tissue ; similarly also 
in the cortex of the root. In this 
family, especially in the parenchyma 
of the root of Sanguinaria canadensis, 
the origin of the laticiferous vessels 
from the coalescence of rows of cells 
may, according to Hanstein, be proved ; 
owing to their imperfect union the re- 
sulting tubes appear moniliform. In Aroideae laticiferous vessels united into a net-work 
occur in the fibro-vascular bundles and the fundamental tissue; but some genera, as 
Caladium and Arum, also exhibit the peculiarity of laticiferous tubes running within 
the xylem, which, from their position, and to a certain extent also from their struc- 
ture, must be regarded as metamorphosed spiral vessels. Simple broad tubes similar 
to these also traverse the fundamental tissue. In the genus Acer the sieve-tubes are 
transformed into laticiferous vessels, as may be seen from their position in the phloem 
and the structure of their wall. 
True laticiferous vessels scarcely occur among Monocotyledons. The peculiar and 
I-IG. T2.—A tangential longitudmal section through the pliloem 
of the root of Scorzoitera hispayiica ; a number of laticiferous 
vessels, anastomosing laterally with one another, traverse the 
parenchymatous tissue ; B a small piece of a laticiferous vessel with 
the adjoining parenchymatous cells, more strongly magnified. 
