Hill . — The Histology of the Sieve-Tubes of Angio sperms. 281 
processes which bring about the secondary modifications of the threads them- 
selves. The real nature of continuity also requires further consideration 
in the light of the facts which have been detailed. 
It is important, in the first place, to notice that the callus in the phloem 
is found only in the sieve-tubes, and that its inception appears to be due 
to the action of ferments on the cellulose walls in the immediate neighbour- 
hood of the threads. From what has been said above (cf. pp. 267 and 279), 
it is clear that the influence of any developing sieve-tube does not extend 
beyond the actual limits of that element, that is as far as the middle lamella 
in any direction, for the callus formation accompanying the boring out of 
the threads always commences at the inner surface of the wall and proceeds 
outwards as far as the lamella. No cases have been observed of the ferment 
action from one tube passing across the lamella into the next, but each 
element behaves as though it is an isolated and independent organism, and 
the formation of the active slime-strings of a sieve-field is due to influences 
emanating from two different and distinct sieve-tubes. The unsymmetrical 
thread-groups between the sieve-tubes and the companion cells and paren- 
chyma cells respectively are thus easy of explanation. For since the sieve- 
tube influence cannot be exerted beyond the lamella, the original proto- 
plasmic threads remain more or less unaltered in the walls of the cells 
(companion or bast-parenchyma cells) to which they belong. 
What, then, is the nature of continuity of protoplasm ? Does the middle 
lamella, representing the original wall laid down after nuclear division, form 
a delicate imperforate membrane against which the ends of the threads 
impinge coincidently from the cells on either side, 1 so that, with Strasburger, 2 
continuity must be considered as rather apparent than real ? Or are the 
threads really continuous across the lamella from cell to cell ? 
The facts of slime-string development appear to lend support to the 
idea of discontinuity, and it might be held that it is owing to the need 
of establishing some form of actual continuity that the complex changes, 
already described, have been evolved. 
If this be the case, then true continuity exists only in the phloem, 
between the sieve-tubes, and possibly between sieve-tubes and the other 
elements, both companion cells and bast parenchyma cells, with which there 
are connecting-threads. The connecting-threads in all other — i.e. extra 
phloem — tissues would, on this view, be little more than very fine pit 
fillings, separated by a pit-closing membrane formed by the middle lamella. 
It is unfortunate that the origin of the threads and their relation to the 
1 The idea of discontinuity was first suggested by Gardiner, v. Roy. Soc. Proc., lxvi, 1900, 
p. 187. Also cf. Proc. Camb. Phil. Soc., xiv, 1907, p. 209. 
2 Strasburger, Jahrb. f. wiss. Rot., Bd. xxxvi, 1901, pp. 502, 503. It is of interest to notice 
that A. Meyer, in his review of this paper of Strasburger’s, Bot. Zeit., 1902, pp. 102-106, is unable 
to accept his views on the formation of the connecting-threads. His objections, however, are based 
on points different to those which have influenced Gardiner in arriving at his conclusions. 
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