FORMS AND SYSTEMS OF TISSUES. 



87 



Papayaceae {Carica and Vasconcelld) 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 tenninate, 

 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 hollow 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 Papaveraceae 

 {Cbelidonium, Papa-ver, Sanguinaria) 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 parenchyma- 

 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 ylcer 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 



Fig. T2.—A tanjfential long-itudinal section through the phloem 

 of the root of Scorzonera hispanica ; 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. 



