605 
Sieve-Tubes of Pinus. 
all stages of which can be followed, is sufficient evidence to 
show that something has travelled along it ; but the discovery, 
that this change can apparently proceed from a sieve-tube 
only as far as the middle of each thread 1 , seems rather to 
point to the absence of any real and continuous communication 
between developing sieve-tubes by means of the connecting 
threads of the young sieve-plates. 
A reason then for the boring out of the connecting threads 
by ferments seems naturally to suggest itself, namely, that 
open canals or pores may be formed which shall afford a 
means of direct and rapid communication between adjoining 
sieve-tubes. Whether the theory that there is no continuous 
intercommunication along a connecting thread is, in reality, 
a fact, and whether it applies to all connecting threads is 
a matter of extreme difficulty to investigate owing to the 
minuteness of the problem involved ; but even should there 
be a minute imperforate membrane separating the two halves 
of a connecting thread, it would be the thinnest portion of the 
pit-closing membrane across which diffusion could rapidly take 
place. 
The Value of the Callus, 
The callus has been thought by some writers 2 to be the most 
important and essential part of the sieve-plate, for the rods of 
callus were considered as the actual connecting elements; but 
it seems to havebeen established beyond a doubt from thepresent 
researches that the slime-strings, which were first noticed by 
Russow, afford the true and only means of direct intercommuni- 
cation between adjoining sieve-tubes. It remains then to be seen 
if any explanation can be offered to account for the presence 
of the callus-rods, and whether any function can be assigned to 
these conspicuous and invariable associates of the slime-strings. 
The origin of the callus-rod, which is due to the action of 
the ferment on the cellulose cell-wall and of the median 
1 See back, pp. 589, 595, 600-2, on the development of the sieves between sieve- 
tubes, and between sieve-tubes and the albuminous cells. 
2 Janczewski, loc. cit. Strasburger, ‘ Leitungsbahnen,’ 1891, and ‘ Plasmaver- 
bindungen’ ; Jahrb. f. wiss. Bot., 1901, 
