288 Hill . — The Histology of the Sieve- Titles of Angiosperms. 
Fig. 24. A field slightly older, seen in section and partly in surface view. The callus-change 
has proceeded further along the threads towards the lamella from the one side, giving the appearance 
of short rods. Note also the protoplasmic pit-filling, p. x 750. 
Fig. 25. A young sieve-field, showing callus-formation on both sides of the wall. The median 
dots are clearly seen at the middle lamella, x 750. 
PLATE XVII (Figs. 26-29). 
Vis cum album . 
Fig. 26. A piece of a sieve-tube in longitudinal section, with a sieve-plate crossed by slime- 
strings. The callus, which is present in the sieve-plate, is not indicated. The lateral wall shows 
protoplasmic threads in pits connecting two sieve-tubes. There is no callus associated with these 
threads, x t,ooo. 
Fig. 27. A patch of sieve-tubes and companion cells in transverse section. The lateral walls 
are crowded with threads in shallow pits. Groups of threads between two parenchyma cells and 
between parenchyma cells and a companion cell (b.p.c.) are also seen, x 750. 
Phaseolus multiflorus. 
Fig. 28. A sieve-tube in transverse section showing a layer of callus laid down all over the wall ; 
also a portion of a sieve-field with slime-strings and callus-pads, x 750. 
Wistaria chinensis. 
Fig. 29. A sieve-tube in transverse section showing a large pad of callus against the wall 
between the sieve-tube and the companion cell. Winter condition, x 750. 
PLATE XVIII (Figs. 30-58). 
Vitis vinifera. 
Fig. 30. A young sieve-plate in longitudinal section, showing the paired callus-basins almost in 
contact and traces of threads at their bases. In some cases the large slime-strings have just been 
formed. An optical section of one of these callus-basins is figured in which four threads are 
seen, x 1,000. 
Fig. 31. A young sieve-plate showing the paired callus-basins with groups of darkly staining 
threads crossing the membrane between them, x 1,000. (Cf. Text-fig. 7, p. 27.) 
Fig. 32. A young sieve-plate with paired callus-basins. There is only a single thread in each 
pit, which is converted into a slime-string, x 1,000. (Cf. Text-fig. 11, p. 30.) 
Fig* 33* A similar plate in surface view, showing a small slime-string in the centre of each 
callus-area, x 750. 
Fig* 34. A young plate of the ‘ single-thread ’ type just bored through. The callus of opposed 
basins is scarcely in contact, and a large median swelling is seen on each string, x 1,000. 
Fig. 35. A plate in a slightly older condition ; the callus now forms continuous rods across the 
plate, and the slime-strings are of the same diameter throughout, x 1,000. (Cf. Text-fig. 1 2, p. 30.) 
Fig. 36. A portion of a similar plate in surface view, showing a fairly large slime-string in the 
centre of each polygonal callus-area, x 1,000. 
Fig. 37. A very young lateral thread group in section in the tangential wall of a sieve-tube just 
outside the cambium. There is no trace of any callus. The lower side is towards the cambium, 
x 1,000. 
Fig. 38. A similar thread-group in a young radial wall. The pit membrane shows slight 
secondary pitting, and callus-heads have been formed at the ends of the threads on one side of the 
membrane, x 1,000. 
Fig. 39. A tangential wall slightly older than Fig. 38. The callus has extended almost to the 
middle lamella from one margin of the membrane, and is just commencing on the other margin. 
The median dots have made their appearance at the lamella, x 1,000. 
Fig. 40. A young sieve-field in section, somewhat overswollen on one side. One-half of the 
wall shows slime-strings in their callus-rods, and nodes at the lamella, whilst the other, younger 
half shows threads and heads of callus with the commencement of slime-strings, x 1,000. 
Fig. 41. A sieve-field in the winter condition after treatment with Acid alcohol and Ammonia, 
and staining by the ordinary Safranin, Water-blue method. Only callus and median nodes are left. 
The callus-rods reach to the nodes, but do not touch each other, x 1,000. 
