2 jo Hill . — -The Histology of the Sieve-Tubes of Angiosperms . 
membrane shows a definitely reticulated structure in surface view before 
there is any trace of callus formation. 
In all cases the protoplasm is seen to have pit processes which occupy 
each depression of the young sieve-plate (Figs. 2 and 3, PL XVII) in a manner 
precisely similar to that of the pit-fillings of normally pitted tissues. Very 
rarely traces of delicate connecting-threads have been seen crossing the thin 
pit-closing membranes of these youngest sieve-plates, and there seems little 
reason to doubt that they are traversed by groups of fine threads like those 
seen crossing the pit membranes between other forms of tissue (cf. Text- 
fig. 6, p. 271). It seems probable that the development of the sieve-plate 
may not take place along quite the same lines either in different plants or in 
the same plant at different times of the year. 
Wistaria . 
Before discussing the nature or cause of this variation, a description of 
the mode of formation of the sieve-plate in Wistaria will be given, since 
some of the clearest and most definite results have been obtained from 
the study of this plant. The general appearance of the horizontally placed 
end-wall of the young sieve-tube — the embryonic sieve-plate — has already 
been noticed (cf. Fig. 1, PI. XVII, and Text-fig. 6). 
Protoplasmic threads have not been successfully demonstrated in the 
very youngest stages, and even when the pits of the sieve-plate show their 
callus linings no certain proof of threads crossing the cellulose membrane 
has been obtained. There is very good reason, however, to believe that 
there is a group of protoplasmic threads in each of these membranes, but 
definite proof of their existence does not seem possible by the methods at 
present at our command. 1 
In sieve-plates showing the paired callus-basins, the protoplasm of the 
sieve-tube may be seen forming little rounded pit processes or fillings 
which fit into the pits of the young plate, and a close examination of these 
processes when seen in section reveals the presence of little papillae at their 
base (Figs. 2 and 3, PI. XVII). The protoplasm of the sieve-tube is always 
slightly contracted by fixation, and so also are the pit fillings ; the appear- 
ance just described appears to be due therefore to the retraction of the little 
papillae from definite holes in the floor of the callus-basins. This view is 
confirmed by a surface view of a young plate, for a group of small round holes 
may be seen occupying the central portion of each polygonal callus-area 
1 Strasburger (Jahrb. f. wiss. Bot., Bd, xxxvi), working with Alcohol material, states that 
he has seen the fine protoplasmic threads in the pit inembranes of the very young sieve-plates, and 
he gives two figures (v. PI. XIV, Figs. 29 and 30) in illustration of his views. It is hardly possible, 
however, to trace any correspondence between hjs figures of the surface view and the section of 
similar sieve-plates, nor do his figures of the next older stage in section and surface view (Figs. 31 
and 32) enable one to arrive at a clear idea of his views as to the construction of the young 
sieve-plate. 
