PLANT TISSUES 
77 
Laticiferous vessels are long simple or branching tubes, which owe 
their origin to chains of superimposed cells whose transverse walls 
have early become absorbed, the lumina of the cells then becoming 
filled with latex. They are found in various parts of roots, stems, 
and leaves. When branched the branches connect with those of 
other tubes forming anastomosing 
network. These vessels occur in the 
following families: Composites, Papa- 
veraceee, Campanulacece, Convolvula- 
cece, Euphorbiacece, Aracece, Oleacece, 
Geramacece, and Musacece. 
Secretory cells with a latex-like 
content are probably of secondary 
origin in plants. They resemble in 
many respects latex cells and are seen 
in various species of the Celastracece, 
Urticacece, Tiliacece, and Oleacece 
families. 
All laticiferous elements contain a 
colorless, milky-white, or otherwise 
colored emulsion of gum-resins, fat, 
wax, caoutchouc and in some cases, 
alkaloids, tannins, salts, ferments, etc. 
This emulsion is called “latex.” 
SIEVE (LEPTOME OR CRIBIFORM) 
TISSUE 
This tissue found in the phloem 
(rarely in the xylem) region of fibro- 
Fig. 27.- — Stages in the develop- 
ment of sieve tubes, companion 
cells, and phloem parenchyma. A, 
a and b, Two rows of plerome cells; 
in c and d, a has divided longitudi- 
nally and c is to become companion 
cells; d, a sieve tube, and b, phloem 
parenchyma. B, c, Companion 
cells, and d, a beginning sieve tube 
from c and d, respectively in A. 
The cross- walls in d are pitted; b, 
phloem parenchyma grown larger 
vascular bundles consists of superim- than in A. C, The same as B with 
, , . j , , t n 1 the pits in the cross-walls of the 
posed, elongated, tubular cells whose sieve tubes become perforations, 
longitudinal walls are thin and com- and the nuclei gone from the cells 
, r ,,, , , , composingthe tube. (From 
posed of cellulose and whose trans- stevens.) 
verse walls, called “ sieve plates ” are 
perforated, permitting of the passage of proteids from one cell to 
another. Occasionally sieve plates are formed on the longitudinal 
walls. Sieve tubes are usually accompanied by companion cells ex- 
