266 
Notes . 
With the increase in thickness of the callus-masses the slime- 
strings become progressively attenuated, and in the case of those 
sieve-tubes which function for a year only the callus finally blocks 
up the pores leading from one sieve-tube to another. 
Connecting threads also occur in some abundance between sieve- 
tubes and other elements of the phloem. Between the sieve-tubes 
and their companion cells (as Gardiner and Hill had already observed) 
threads are very numerous and very short, for the cell-walls are 
furnished with a great number of deep pits elongated in the horizontal 
direction. 
Sieve-tubes are placed in communication with adjacent bast-paren- 
chyma cells by threads, which are in some cases fairly numerous and 
are usually short and occur in small and deep pits. 
Cells comparable in the structure of their threads to the albuminous 
cells of Pinus sylvestris , &c., apparently occur in the phloem of Vitis 
vinifera. 
In the cases just cited the groups of threads are covered in winter 
by callus-pads, which however are formed only on the sieve-tube side 
of the groups, and the connecting threads can usually be seen to 
traverse the callus-masses. 
It is interesting in this connexion to notice that sieve-tubes appear 
to be the only elements of the bast in which callus is formed. 
The development of the terminal sieve-plates is very difficult to 
investigate, owing to the thinness and delicacy of the pit-closing 
membrane, but the history of the lateral sieves can be more easily 
followed. The development of the sieves in the radial and tangential 
walls appears to be analogous to the development of the sieves in the 
radial walls of the sieve-tubes of species of Pinus , and the sieves in 
the radial and tangential walls of most Angiospermous sieve-tubes 
appear to pass through similar developmental stages to those which 
have been described for such Gymnosperms as Pinus. Groups of 
fine threads can be seen in the lateral walls of the youngest sieve- 
tubes, which by the action of ferments (as it would appear) are bored 
out and converted into slime-strings, the cellulose membrane in the 
immediate vicinity being at the same time altered into callus. 
In this way the callus-rod enclosing a small group of slime-strings 
is produced. 
All stages in the process have been seen. The precise mode of 
development of the end-wall sieve-plates has not yet been seen very 
