284 Hill . — 77 ^ Histology of the Sieve-Tubes of A ngio sperms. 
more than one year, the slime-strings are re-formed in the spring along the 
old paths by renewed ferment activity. The accumulated callus is dissolved 
away and the plate resumes its normally active appearance. 
9. With the disuse and death of the sieve-tube the whole of the callus 
is dissolved and an open sieve remains which represents the cellulose frame- 
work or non-pitted portion of the embryonic sieve-plate. 
10. The sieve-fields in the lateral walls between two sieve-tubes show 
a similar origin to that of the sieve-plates. In the adult condition fine 
slime-strings, each in a callus-rod, cross the pit-closing membrane. In 
these cases it is probable that ferments from the sieve-tubes on either side 
of the common membrane, acting along the original protoplasmic threads, 
convert them into slime-strings and at the same time produce the callus- 
change in the cellulose immediately contiguous to the threads. In this 
way the single string in each callus-rod, characteristic of the Angiospermous 
sieve-fields examined, is produced. In the Gymnosperms examined a group 
of fine slime-strings was found in each callus-rod. 
11. The activities of the sieve-tube contents appear to be limited by 
the outer boundary of the cell-wall, i. e., the middle lamella, since ferment 
action is never seen to cross the lamella, but always to proceed as far as this 
point from each individual tube. It seems likely that similar restrictions 
may apply to all cells. 
12. The callus-rods are not continuous across the common membrane, 
but are formed of two distinct half-rods, separated by a median node. 
13. The median nodes, which are found at the middle lamella 
separating the opposed halves of the callus-rods of the sieve-fields, appear 
to be the product of ferment action on the substance of the middle lamella. 
In Viscmn album , where in the sieve-fields no callus is formed, the median 
nodes are also absent. 
14. These median nodes enclose the nodules or nodal swellings which 
represent the points of origin of the protoplasmic threads ; but whether the 
nodal points are halved by a fine separating lamella (thus preventing actual 
continuity between cells) is uncertain. 
15. Callus-pads may be formed over the lateral sieve-fields, apparently 
by protoplasmic activity, in the same way as over the sieve-plates, and may 
be re-opened in a similar manner. 
16. Callus does not always accompany the threads or strings of the 
sieve-fields ; it is absent from the sieve-fields of the lateral walls in Viscum 
album. An analogous case may be afforded by the albuminous cells in the 
leaf of Pinus sylvestris. 
17. Between sieve-tubes and companion cells, thread-groups, compar- 
able to those described between the albuminous cells and the sieve-tubes 
of the stem of certain Gymnosperms, appear to exist. On the sieve-tube 
side of the common membrane the threads, at first protoplasmic, are con- 
