579 
Sieve- Tubes of Finns . 
covered by brown cushions, which, when examined in optical 
section, allow the callus-rods *to be seen as dark patches on 
a clearer ground. The smaller the overlying callus-cushions, 
the more easily can the outlines of the callus-rods be seen 
through them (Fig. 2, PI. XXXI). 
He then discusses the contents of the tubes, and draws 
.attention to the staining of the starch, which becomes a 
reddish-violet colour, whilst in the parenchyma it is a black 
purple ; the pale staining is due to the presence of ferments, 
for starch stains in this manner in places where ferments are 
known to occur. This interesting observation will be referred 
to again later 1 . 
In his description of the development of the sieve-tubes he 
notices the primordial pits throughout the cambium, and 
that on the phloem side near the youngest mature sieve-tubes 
(the boundary- cells 2 ) the surfaces of the pits are strewed with 
very fine points ; he never, however, sees any * threads ’ in 
a section of such a developing sieve-plate, but in the first 
thick -walled sieve-tube he notices yellow striae crossing the 
pit-closing membrane. Passing outward the callus appears 
in the second or third mature sieve-tube, filling the crenu- 
lations (or secondary pits) of the pit-closing membrane 
(Fig. 8, PL XXXI, c.), and stretching in towards the interior as 
very fine, often double brown stripes (Fig. 9, PL XXXI, gs). 
In the next tube the stripes have become much thicker and 
now form the callus-rods whose extremities enlarge to form 
the callus-heads or cushions (Fig. no, PL XXXI, c.). About 
or before this time the median nodules are first noticeable. 
Nothing is said about the origin of the rod lets — i. e. the slime- 
strings — which are enclosed in the callus-rod. 
A further contribution to the subject of sieve-tubes is made 
by Russow 3 in his well-known paper, ‘ On the perforation of 
the Cell-wall ■ ; for by the use of the methods there described 
1 Cp. P- 594 - 
3 The term ‘ boundary-cells ’ was used by Russow to designate the youngest 
layer of active sieve-tubes. 
3 Russow, Sitzber. der Dorpater Nat.-Ges., 1883, p. 570. 
Q q 
