u 
Hill . — The Histology of the Sieve-Tubes of A ngiosperms. 271 
(Fig. 4 , PI. XVII, and Text-fig. 7). Into these holes or tubes the papillae of 
the pit processes fit, and in some instances portions of these papillae may be 
seen in the actual holes in a surface view of such a plate. Owing to the 
necessary thickness of the sections it is practically impossible to distinguish 
these little pores in a transverse section of a sieve-plate, for there must be 
a certain amount of (blue staining) callus either at the back or the front of 
a pore by which it is hidden from view. In the next stage the paired pit 
processes on either side of the thin membrane are found to be united by a 
group of short darkly-staining threads or slime-strings, and the callus extends 
across the membrane. In some cases the small strings are very short and 
Fig. 7. 
Diagrammatic sketch to show the development of the slime-strings of the sieve-plate from 
groups of threads : — 
Fig. 6. Shows the deeply pitted plate or end- wall, with connecting-threads {p. t.) crossing the 
pit-closing membrane {cl.) and uniting the protoplasm {p.) of adjoining sieve-tubes. 
Fig. 7. A later stage ; ferment action has commenced, the threads are being bored out to 
become small slime-strings {s. s.), and this is accompanied by the alteration of the surface layers 
of the membrane {cl.) to callus {c.). The final stage is the fusion of the small slime-strings to 
one large string (cf. Figs. 2-4, PI. XVII). 
Fig. 8. The active sieve-plate almost completely developed at a ; a group of slime-strings 
is seen crossing the pit-closing membrane, which is now converted entirely into callus {c.), whilst at 
the other end of the plate single thick slime-strings are seen, due to the confluence of the group of 
strings, each consisting of a mucous string ensheathed in its protoplasmic tube ; protoplasm and 
slime are not differentiated in this figure (cf. Figs. 2 and 3, p. 262, and Figs. 5-9, PI. XVII). 
difficult to distinguish (Fig. 8, PI. XVII), whilst in others they are of a 
fair length and the whole group can be seen to be definitely ensheathed in 
callus (Figs. 6 and 7, PL XVII). The mesh of the sieve-plate retains its 
cellulose character, but is everywhere coated with callus. 
These groups of small slime-strings, which on the analogy of the sieve- 
fields must be due to the boring out and enlargement of pre-existing con- 
necting-threads, have as a rule only an ephemeral existence ; for the little 
membrane, now converted into callus — through which each group passes — 
is rapidly disorganized, with the result that a large open hole is formed 
