PLANT TISSUES 133 
roots, stems, and leaves. The branches connect with those of 
other tubes forming an anastomosing network. ‘These vessels 
occur in the following families: Composite, Papaveracee, Campanu- 
laceea, Convolvulacea, Euphorbiaceae, Aracee, Oleaceea, Geraniacea, 
and Musacee. 
(3) Ca@NocYTIG LATEX CELLS are elongated, multinucleate, 
laticiferous cells which occur in the milkweed family (Asclepiada- 
cee) and in Euphorbia species. ‘They arise, in each instance, from 
a single cell in the embryo which elongates at an equal rate with 
the growth of the plant, extending itself between other cells like a 
filamentous hypha of a mold, branching freely, but unlike 
laticiferous vessels the branches do not anastomose. 
All of the latex elements are living, possessing thin layers of 
protoplasm just within their walls and containing one or more 
nuclei depending upon the type, also at times, /eucoplastids, 
elaioplastids and proteinoplastids. The leucoplastids form starch 
grains, the elaioplastids build up oil globules and the proteino- 
plastids organize protein granules. 
The exact role of latex elements remains undiscovered. 
Some botanists believe them to be food reservoirs, others excre- 
tory reservoirs, while still others regard them as representing a 
conducting system for the movement of starch and proteins. 
All lacticiferous 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 ‘“‘/atex.”’ For further details see under ‘‘Latex”’ 
in Chapter V. 
Sreve (LEPTOME OR CRIBIFORM) Tissur 
This tissue, found in the phloem region of fibro-vascular 
bundles, is a product first of the procambium and later of the 
cambium. It consists of superimposed, elongated, tubular 
cells whose longitudinal walls are thin and composed of cellulose 
and whose oblique or transverse walls, called “sieve plates,” are 
thickened and perforated, permitting of the passage of proteids, 
amino-acids and amides from one cell to another. Frequently 
sieve plates are formed on the longitudinal walls, as in Cascara 
Sagrada. The mature sieve tube has a thin layer of cytoplasm 
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