Cuscuta and its Host . 
659 
In the youngest stage the transverse wall of the young sieve tube is 
traversed by a number of faintly staining threads, on which a median 
thickening is sometimes visible. 1 The wall is commonly unpitted. In 
Fig. 19 the pitting is more marked than is usual in so young a trans¬ 
verse wall, and the threads are exceptionally crowded. The threads soon 
stain more darkly, and at the same time specks of callus appear at their 
ends, very speedily spreading along them (Fig. 20). Owing to the 
small dimensions of the plates, it is difficult to follow the details of this 
process, but it can be seen that at the next stage each thread stains very 
darkly with protoplasmic stains, and is enclosed in a tube of callus. In 
a few cases a median nodule 2 has been distinguished on the callus rod, 
which is thus seen to be separated into two plugs. (Cf. Fig. 20 B.) 
Even at this stage the sieve plate is only very slightly pitted ; as a rule 
a single thread or slime string is found in each pit. Fig. 21 shows groups 
of strings occurring together in small depressions, and is taken from a sieve 
tube of C. europaea. 
The formation of callus next spreads over the region in between the pits, 
and the sieve plate is now fully mature (Figs. 22 and 23). In surface view 
the perforations are easily demonstrated, each pore being lined with callus 
and traversed by a single slime string (Figs. 25 and 26). In the material of 
C. europaea , preserved out of doors in October, some accumulation of callus 
had taken place over the sieve plates, but very little was present in green¬ 
house material of C. reflexa in October (Figs. 24 and 27). In C. rejiexa growing 
on Ficus in the greenhouse in November, large masses of callus had accumu¬ 
lated. Mirande 3 has also figured still larger masses of callus in other species. 
L ateral walls of the sieve tubes . Lateral sieve plates were only rarely 
found ; in them all the phases in the development of slime strings and callus 
rods appear to be similar to those in the terminal sieve plate. Fig. 27 
represents a lateral sieve plate, against which a large accumulation of callus 
has taken place. 4 Short strands of cells with sieve plates on their trans¬ 
verse walls are occasionally found connecting two strands of sieve tubes; 
such a connecting element is shown in Fig. 28. 
Groups of threads appear to be equally numerous in the tangential and 
radial walls of the young sieve tubes. 6 Sometimes each group contains 
very few threads, usually in a small pit (Fig. 30, C. europaeus) ; sometimes 
a large group stretches across the sieve-tube wall, forming a wider 4 sieve 
field ’ (Fig. 29, C. reflexa). 
The development of the threads in the lateral walls, each into a slime 
1 Cf. Hill, a and b, 1901 ; 1908. Sykes, 1908. 
2 Cf. Hill, 1901 b, Figs. 20 and 21, PI. XXXIII; 1908, Figs. 34, &c., PI. XVIII. 
3 Mirande, p. 88 and Fig. 7. 
4 From C. europaeus on Vilis, October; cf. Peirce, 1893, p. 294. 
5 For a more detailed description of the distribution of pores in the lateral walls, see Mirande, 
pp. 85-7, and Figs. 6 and 7. 
