Hill . — The Histology of the Sieve-Tubes of A ngio sperms. 261 
four or five years, and if remounted in a medium containing Iodine they will 
regain their former sharpness of detail. As in the case of Pinus , the material 
has been studied as much as possible by the use of transverse and radial 
sections. By this means the various stages in the development of the sieve- 
plate can be more readily followed, and mistakes can be avoided such as 
may easily be made when examining tangential sections of unknown age 
by the Acid Blue method. 
The Active Sieve-Tube. 
So much has been written about the structure of sieve-tubes, with so 
many conflicting details, that it may help to 
make what is to follow more clear if a short 
account of the structure of the active sieve- 
tube is given by way of preface. 
The active sieve-tube may be regarded 
from two points of view, namely, with reference 
to the structure of the walls of the tube, and 
also as to the nature of the contents. Without 
going into details it may be stated that the 
contents consist of a hollow sac-like proto- 
plasmic body in intimate communication with 
the protoplasmic bodies of neighbouring sieve- 
tubes, in the vertical direction by means of the 
large pores in the sieve-plates, and laterally by 
means of the finer connecting-threads or strings, 
and occasionally also by lateral sieve-plates. 
The protoplasmic body encloses the peculiar 
slimy contents, consisting of metabolized ma- 
terial in a state of flux, which give the 
sieve-tubes their characteristic appearance 
in fixed material. The slimy contents pass 
through the pores of the sieve-plate from 
one sieve-tube to the next — the slime-strings 
— but they do not come into contact with the 
cell-wall, owing to their enclosure in proto- 
plasmic tubules which pass with them through 
the holes in the sieve-plate. A similar struc- 
ture, but on a much smaller scale, appears 
to be shown by the groups of lateral con- 
nexions. 
The cell-walls of the sieve-tubes — at least in Vitis and Wistaria — do 
not appear to be sharply differentiated in their chemical composition from 
other cell-walls of the phloem, though according to their staining reactions 
Fig. i. A young sieve-tube 
( s.t .) in longitudinal section, show- 
ing the sieve-plates {s. p.) crossed 
by slime- strings. The tube shows 
the slimy contents ( s .) within the 
protoplasmic sac ( p.s .). On one 
side a companion cell ( c . c.) is seen. 
(Copied from Wilhelm, PI. II, 
Fig. 13, Vitis vinifera . ) 
