Hill . — The Histology of the Sieve-Tubes of Angio sperms. 267 
Blue, tiny specks or dots of callus will probably be seen at the end of each 
thread on one side of the membrane, or possibly on both surfaces (Fig. 38, 
PI. XVIII). With increasing age the thickened portions of the wall become 
more conspicuous, and the smaller groups of threads also tend to become 
arranged in small secondary pits (Fig. 39, PI. XVIII). Whilst this change in 
the general configuration of the cell-walls has been taking place, more 
remarkable changes are taking place in the character of the threads, and in 
the wall in their immediate environment. The callus has now reached the 
middle lamella, usually at first from one side only (Fig. 39, PI. XVIII), and 
then shortly afterwards from the adjoining element, so that each thread 
shows a callus coating (appearing as a blue rod with Water Blue alone), 
which has been formed by an alteration of the cell-walls of the two 
sieve-tubes concerned, proceeding inwards as far as the middle lamella 
(Fig. 40, PL XVIII). 
By slightly swelling the walls and treating the section with either 
Russow’s Callus Reagent or Water Blue, the callus-rods, which appear at 
first sight to be continuous across the membrane, are seen to be made up 
of two distinct portions or pegs belonging to the walls of their respective 
sieve-tubes, and apparently there is no fusion or actual contact between 
them (cf. Figs. 12 and 13, PI. XVII). 
With the appearance of the callus-rods a further structure becomes 
visible, namely, a median node or dot, situated between the two opposed 
pegs of the callus-rod, which is stained with protoplasmic dyes (Fig. 16, 
PI. XVII). The formation of the callus-rods is, however, only the obvious 
sign of a more important change, which has taken place in the character 
of the threads themselves. Ferments — which, as will be shown later, also 
determine the structure of the developing sieve-plate — appear to attack the 
fine threads of these lateral sieve-fields, and bore out the holes or capillaries 
in the cell-wall through which they pass from cell to cell. 1 So then not 
only do the ferments, which pass in from each sieve-tube, enlarge the thread 
canals, but they also alter the nature of the wall in the immediate neigh- 
bourhood, by converting it into callus as far as the middle lamella. The 
contents of these fine tubes are found to react in a different manner to 
stains after the appearance of the callus, for the protoplasmic threads of the 
embryonic condition have now become the more important slime-strings 
of the mature sieve-tube, and in addition to this the small darkly staining 
node has been formed at the middle lamella. Each slime-string is thus 
seen to be contained in its own callus-rod or tube, and there is no such 
inclusion of several strings in a single callus c cork ’ as has been described 
for Pimis 2 and other Gymnosperms. In Wistaria the development of the 
1 Cf. Hill, 1 . c., p. 588. The probable role of ferment action in the production of the slime- 
strings and callus-rods was suggested by Gardiner, and he drew my attention to the analogous 
appearances presented by the endosperm walls of Tamus communis during germination. 
2 Ibid., p. 590. 
T 2 
