607 
Sieve-Tubes of Pinus. 
the structure of the sieve-plate can be attributed to the 
unsatisfactory nature of the reagents employed, since they do 
not differentiate between protoplasmic and callus structures ; 
and for this reason the development of the sieve-plate could 
not be understood. 
The youngest sieve-plates which could be examined showed 
‘ connecting threads * like those in parenchymatous tissues ; 
but in the so-called ‘boundary cells’ — i. e. the youngest 
thick-walled sieve-tubes — a change takes place ; for a ferment 
apparently travels along and bores out the threads, converting 
them into slime-strings, and at the same time so affects the 
surrounding cellulose that the formation of the callus-rods 
results ; and at the middle lamella, owing to the same cause, 
the median nodule appears enclosing the nodes of the slime- 
strings. 
The effect produced by the action of ferments on the threads 
in the endosperm-walls of germinating seeds of Tamus is 
described, and its similarity to the state of affairs found in the 
developing sieve-plates of Pinus is pointed out, for the two 
cases appear to be analogous. 
The development of the sieves between the albuminous 
cells and the sieve-tubes has also been worked out, and it has 
been found that the portions of the threads on the sieve-tube 
side of the middle lamella undergo changes precisely similar 
to those described for the sieve-plates, whilst the shorter 
portions on the cell-side of the lamella retain their proto- 
plasmic character. 
The two rival theories as to the formation of the callus are 
also considered, and reasons are given for supposing that it 
may arise either by mucilaginous degeneration of the cell-wall 
or by deposition from the protoplasm. 
The paper closes with a discussion as to the value of 
‘connecting threads’ for translocation, about which it is 
difficult to come to a definite conclusion ; the slime-strings, 
however, do seem without a doubt to be of great importance 
as translocatory channels between adjoining sieve-tubes. 
In conclusion, it is suggested that the callus maybe of value 
