154 



LATICIFEROUS VESSELS 



with thick white walls. Longitudinal sections, cut tangentially 

 to the phloem, show the characteristic form of the tubes, and 

 branching can often be recognised (Fig. 77, B). The granular 

 latex, which has been coagulated by the preservative (spirit), 

 contains curious starch-grains somewhat resembling minute 

 knuckle-bones. These, and the thick walls of the tubes, are 

 peculiar to the Spurges, but in other respects the features just 

 described are applicable to all laticiferous cells. 



By contrast laticiferous vessels, which are characteristic of 

 Papaveraceae, Campanulaceae, and Compositae, are formed from 

 rows of cells (which may run in any direction, though preva- 

 lently longitudinal) 

 by the partial, or 

 usually complete, 

 breaking down of the 

 cross-walls. Latici- 

 ferous vessels are 

 usually not recognis- 

 able in the embryo, 

 but arise at a later 

 stage in development. 

 They too form an 

 extensive system in 

 all parts of the plant, 

 most commonly near 

 or within the phloem. 

 They are readily distinguished from the laticiferous cells, how- 

 ever, by' the occurrence of frequent fusions between their 

 branches, as a result of which they form a highly irregular 

 network (Fig. 78, C). The mode of origin of these elements can 

 seldom be recognised in the adult condition, but in the Greater 

 Celandine (Chelidonium majus) longitudinal sections show remains 

 of the partially absorbed transverse septa quite clearly. 



Laticiferous vessels are abundant in the secondary phloem 

 of the fleshy roots of the Dandelion (Taraxacum) or Salsify 

 (Tragopogon, Fig. 78, A) ; in transverse sections of preserved 

 material they are plainly recognisable by their brown contents. 

 They have comparatively thin walls and present a very irregular 

 shape (Fig. 78, B, I.), which is due tQ the plane, of section often 



FIG. 77. Laticiferous cells (/.) in the stem of 

 a Spurge (Euphorbia) in transverse (A) and 

 longitudinal (B) sections, pa., parenchyma 

 of cortex ; ph., secondary phloem ; xy., 

 secondary xylem. 



