282 Hill . — The Histology of the Sieve-Tubes of A ngio sperms, 
cell-plate have not yet been studied in detail, but there seems very good 
reason for maintaining the views put forward by Gardiner 1 as to the genesis 
of the connecting- threads. On this hypothesis continuity exists ab initio , 
and at the middle lamella are to be found the first beginnings of the 
threads, which eventually may become conspicuous as median dots. 
Certainly the median dot shows the same staining capacity as the rest 
of the thread, and in many cases it is a scarcely noticeable feature . 2 But 
whether these nodes of the protoplasmic threads are entire or are bisected 
by a delicate membrane is somewhat uncertain. 
At present, therefore, there seems to be some ground for doubting 
whether protoplasmic continuity — in the sense of a continuous filament of 
living protoplasm— exists between vegetable cells. 
With regard to the question of the genesis of the connecting-threads, 
some support to Gardiner’s view appears to be given by the distribution and 
character of the threads in the wall between a sieve-tube and its companion 
cell. As is well known, these two cells arise by division from a common 
mother cell, and the wall between them is crowded with threads. When the 
elements are young, the threads are distributed more or less uniformly 
throughout the wall, and their arrangement suggests strongly that they have 
originated in connexion with the processes of nuclear division , 3 and may 
represent the nodes of the spindle fibres around which it seems probable 
that the primitive cell-wall is laid down . 4 
With regard to the functions of connecting-threads generally, it seems 
highly unlikely that they serve for anything more than the conveyance of 
impulses from cell to cell. Each cell therefore retains its individuality and 
is master of its own domain, even though it is in sympathetic communication 
with its neighbours. It is thus conceivable that even if a membrane of ex- 
treme tenuity should be found to exist separating the threads of one cell 
from actual contact with those of another, with which they appear to be in 
contact, our conception of the efficacy of the threads, as paths for the 
transmission of stimuli, would not be seriously impaired . 5 
Gardiner’s view of the origin of the threads, however, though it has not 
received definite proof, deserves very careful consideration, since it affords 
a simple solution of the question of the mode of origin of the threads and 
one with which it is easy to accommodate the observed facts both as to the 
1 Gardiner, Roy. Soc. Proc., lxvi, 1900, p. 187 ; Proc. Camb. Phil. Soc., xiv, 1907, p. 209. 
2 Kuhla, Bot. Zeit., 1900, PI. III. A conspicuous median dot is figured on nearly all the 
threads. In Viscum examined by our methods this dot is seldom seen, and its prominence is 
probably due to the method employed by Kuhla. 
3 Cf. Hill, Phil. Trans. Roy. Soc., 1901. 
4 Cf. Gardiner, Roy. Soc. Proc., 1900. 
5 Cf. Hill, Ann. Bot., vol. xv, pp. 603-605, where the views of Pfeffer and of Brown and Escombe 
are discussed. 
