62 ELEMENTS OF PLANT ANATOMY. 
of the bands of membrane between the pores, in all directions, 
and it is sometimes carried so far as entirely to close them. In 
a C 
SOP LPR een 
ry en 
Fig. 30. 
Fragments of sieve-tubes. 
$s sieve-plate in cross-sec- 
tion. s’ seen from the sur- 
face. c callous substance. 
x 300. — ( Wiesner.) 
some instances, 
masses of this cal- 
Sieve-tubes from Acer. sieve- 
plates in @ X 150, in b xX 400 . 
times. a callous substance. p found, covering 
protoplasm. — (Th. Hartig.) 
lous substance are 
the sieve-plate on 
each side, leaving no indications of the 
openings or pores. This substance is 
easily dissolved, and it is believed that 
by its formation the function of the sieve- 
plate is arrested during the period of 
Fic. 29. winter inactivity, to be resumed again in 
Piece of sieve-tube of Vitis Summer, the callous substance being dis- 
vinifera. 8 8 sieve-tubes- solved away.-_ he contentssor maesuu bes 
gg accompanying cells, sp i nie 
compound sieve-plates with are such as to support the opinion that 
eight sieve areas. x 400. — t] ia f ot] x t R d t tds ae 
(Accd. to Withelm.) 1eir function is to conduct various 1n- 
soluble materials in masses from place to 
place in the plant. In their mature condition there is a thin 
layer of protoplasmic substance lining the wall. The remaining 
