SIEVE-TUBES—CAMBIFORM CELLS. 231 
older layers of wood, which are no longer vitally active, while 
the sap-wood is formed of the younger wood, which often com- 
prises but a few annual rings. 
The second component of a vascular bundle is the sieve 
portion’ (phloém in part). This consists of sieve-tubes (with 
latticed cells) and cambiform cells; its elements never become 
lignified, which, on the contrary, occurs regularly with the ele-_ 
ments of the wood. 
The sieve tubes’ are elongated, thin-walled cells, which 
correspond, in their entire structure, to the vessels (in the 
xylem) (Figs. 147, 148). They serve for conducting non-dif- 
fusible, albuminous substances, which they abundantly contain, 
and are, therefore, mostly divided by oblique, transverse plates, 
which appear pierced by numerous holes (sieve-plates, Fig. 
148 w). 
The sievye-tubes are often accompanied by small latticed cells, 
which, from the manner of their development, belong to them. 
The ecambiform cells are elongated, thin-walled cells, very 
rich in protoplasm, without pits, which are very similar in 
shape to the cambium cells, and conduct the more readily diffu- 
sible substances. ‘ 
Other parenchymatous elements besides those already men- 
tioned also participate in the conduction of substances, thus, 
particularly, the starch sheaths and sugar sheaths, which 
surround the vascular bundles. 
In the dicotyledons and gymnosperms, the medullary rays, as 
a rule, also intersect the sieve portion (bark rays or phloém 
rays); often, however, they remain imperceptible in the tissue 
of the bark (Radix Ipecacuanhe).* If they are distinctly 
1 Alsoincorrectly termed ‘‘ soft bast.” The term “ bast” should remain 
confined to the mechanical elements of bast-cells. ae 
2 Wilhelm, ‘ Beitrage zur Kenntniss des Siebréhrenapparates dicotyler 
Pflanzen,” Leipzig, 1880. Janczewski, ‘‘ Vergleichende Untersuchungen 
iiber die Siebrdhren.” Sitzungsber. d. Krakauer Akademie, .881. De 
Bary, ‘‘ Anatomie,” p. 179. 
* Arthur Meyer, Archiv der Pharm., 221 (1883), p. 737. 
