66o Thoday.—On the Histological Relations between 
string enclosed in its callus rod, is similar to the development of the threads 
in the terminal sieve plate; the only difference is that the perforations are 
here much finer. 
Each fully formed slime string thus becomes enclosed in a very minute 
callus rod which is only very slightly bored out (Figs. 30 A and 37 A,P 1 .XLIX). 
The median thickenings are found to be peculiarly conspicuous on 
the young threads in the lateral walls (cf. Figs. 19 and 31) ; a median node 
is also often clearly visible on the mature callus rod (Figs. 32 and 39). 
In Fig; 31 A a very early stage of callus formation is seen, and in one 
group the change has proceeded as far as the middle lamella from one side, 
but has not taken place at all on the other side of the middle lamella. 1 
In the walls of the sieve tubes which abut on a companion cell, numerous 
small pits usually occur, and in these pits groups of threads are found. In 
many cases that half of the thread which is on the sieve-tube side of the wall 
has undergone development into a short slime string enclosed in a callus 
rod, while the half on the side of the wall which belongs to the companion 
cell has remained unchanged 2 (Figs. 19, 24, 41, and 42). 
In the summer material very small accumulations of callus were some¬ 
times formed over the pits in the lateral walls, but in the material pre¬ 
served in October much larger masses were present, which often extended 
over a considerable portion of the intervening wall. Figs. 35 and 39 
represent large groups of slime strings in the lateral walls of two sieve 
tubes, over which small pads of callus have been deposited ; Fig. 38 shows 
in surface view a wall in which many small groups occur, and a small pad 
of callus has been deposited over each group; Fig. 40 represents a wall 
inside which the callus forms an almost continuous layer. The pencilled 
lines in the blue callus indicate the 4 paths J above the slime strings, through 
which continuity with the pores is for a long time maintained. 3 
Sieve tubes of the haustorium. The sieve tubes of the haustorium are 
composed of very short elements, and sieves are numerous in all their walls. 
It appears that any parenchymatous cell may become organized as a sieve 
tube, but companion cells are by no means universal. 
The histology of the haustorial sieve tubes corresponds in nearly all 
respects with that of the sieve tubes of the main stem. It has already 
been mentioned that the threads or slime strings in the transverse 
plates are commonly arranged in groups. As a rule, the callus rods 
traverse the entire thickness of the wall which separates the phloem 
elements. Callus appears to be formed earlier in the haustorium than in 
the main stem; in Cuscuta reflexa large masses of callus were already 
1 Hill, 1901 b, Fig. 18, PL XXXIII, and p. 589; Hill, 1908, Figs. 23, 24, &c., PL XVII, and 
pp. 281, 367; Sykes, 1908, Figs. 14 and 15, PL XIX, and pp. 307, 316. 
2 Hill, 1901, pp. 600-1 ; 1908, pp. 279-80; Sykes, 1908, p. 317. 
3 Hill, 1901; 1908. 
