262 Hill. — The Histology of the Sieve-Tubes of Angiosperms , 
they seem to possess a somewhat higher percentage of cellulose. The sieve- 
plates, or rather the sieve-plate reticulum, at first shows a composition 
similar to that of the lateral walls of the tube, but in the mature condition 
its free surface has be- 
come converted into 
callus, so that in a 
transverse section of 
the plate there is seen 
to be a central core of 
cellulose with a border 
of callus, whilst in a 
surface view the pores 
of the sieve-plate show 
a callus lining (cf. Text- 
figs. 2 and 3). A some- 
what similarly complex 
structure obtains in the 
lateral thread groups, 
for callus is nearly al- 
ways to be found in as- 
sociation with the con- 
nexions between two 
adjoining sieve-tubes. 
A transverse section then of a sieve-plate with its slime-strings would 
show a highly complex structure, for, taking a single string and its pores, 
there would be in the centre the slime 
itself, around which is the protoplasmic 
tubule, and immediately outside comes 
the ring of callus due to the peripheral 
alteration of the cellulose membrane 
of the sieve-plate itself. Such a struc- 
ture appears to be typical for the 
sieve-plates of all Dicotyledons, in- 
cluding Viscum , though according to 
Kuhla’s figures 1 it possesses no callus 
in its sieve-plate. 
With regard to the position of the 
sieve-plate, enough has been written 
by Wilhelm and Lecomte to render 
any lengthy description unnecessary. 
The sieve-plates are commonly placed transversely like ordinary end-walls, 
and they occur in this manner in Wistaria , Cucurbit a , Viscum , and in the 
Fig. 2. Diagrammatic longitudinal section of an active sieve- 
tube of Vitis, showing the cellulose wall and meshwork of the sieve- 
plate in section {cl.). The protoplasmic bag (/.) encloses the 
mucilage or slime ( s .) : the sieve-plate meshwork is coated with a 
layer of callus ( c .), and this in its turn is separated from the 
sieve-tube contents by the protoplasm. Cf. the surface view of a 
similar sieve-plate (Fig. 3). 
Fig. 3. Diagrammatic view of a sieve- 
plate similar to that of Fig. 1, in surface 
view. The mesh of the plate (cl.), with the 
pores lined by callus (r.), are shown, through 
which the slime-strings (s.) pass, ensheathed 
by protoplasm (/.). 
1 Kuhla, 1 . c., PI. Ill, Fig. 17. 
