Hill.— -The Histology of the Sieve- Tubes of A ngio sperms. 289 
Fig. 42. A field from the same material as Fig. 41 after treatment with 10 per cent. Potash for 
two days, and washing in weak Hydrochloric acid ; callus and nodes have vanished and empty tubes 
are seen in the two portions of the split membrane. x 75°* 
Fig. 43. A transverse section of a sieve-tube after the action of Russow’s Iodine solution and 
75 per cent. Sulphuric acid. The walls have turned deep blue and have swollen greatly. Slime- 
strings and protoplasmic structures have vanished, and only the median nodes are seen as highly 
refractive bodies, x 1,000. 
Fig. 44. A lateral wall of a sieve-tube in surface view. The sieve-tubes are composed of small 
groups of strings, and each group is covered by a head of callus formed by the union of the heads of 
the callus-rods which enclose each slime-string, x 75°* 
Fig. 45. A transverse section of the phloem, showing sieve-tubes and companion cells. One of 
the sieve-fields ( a ) in the radial wall has .undergone further boring out to give a normal sieve-plate 
with thick slime-strings, and the field ( b ) shows an enlargement of some of its slime-strings. Small 
thread-groups are seen in the walls between companion cells and both sieve-tubes and bast parenchyma 
cells, x 1,000. 
Fig. 46. A sieve-plate in section. The pit nearest the side-wall is furnished with strings of the 
sieve-field type. The others are ordinary sieve-plates. This sieve-plate belongs to one of the short 
sieve-tubes crossing a medullary ray. x 750. 
Fig 47. A piece of the phloem in tangential section, showing sieve-tubes with an inclined 
compound sieve-plate, a companion cell with its threads, bast parenchyma cells, and a group of 
threads between one such cell and a sieve-tube with the callus only on the sieve-tube side. Sieve- 
fields in surface view, &c. x 75°* 
Fig. 48. A piece of a radial wall of a sieve-tube in transverse section. The slime-strings of the 
sieve-field have been bored out to form definite continuous connexions between the two tubes, 
x 1,000. 
Fig. 49. Wall between a bast parenchyma cell and a sieve-tube, showing groups of threads with 
callus-cushions on the sieve-tube side through which the threads are continued, x 750. 
Fig. 50. A transverse section showing the connexions between two sieve-tubes and a bast 
parenchyma cell. The callus-rods reach as far as the middle lamella on the sieve-tube side only. The 
sieve-tube portion of the wall is the broader. Slime-strings are seen in the callus-rods and there 
are nodes at the lamella. Thread-groups between a companion cell and a sieve-tube are also seen, 
but callus is not indicated, x 1,000, 
Fig. 51. A companion cell in longitudinal section. The threads in the walls between the cell 
and the sieve-tube do not appear to be homogeneous. On the cell side of the wall they are short 
and darkly staining, whilst on the sieve-tube side they are faint and longer and a trace of callus is 
seen on the surface. They are rather of the nature of fine slime-strings. A thread-group between 
the companion cell and a bast parenchyma cell is also shown. X 1,000. 
Fig. 52, Wall between a companion cell and a sieve-tube in winter material. Callus is seen in 
the sieve-tube half of the wall, x 750. 
Fig. 53. Young bast-fibres in transverse section, showing the funnel-like pits with groups of fine 
threads crossing the pit-closing membrane. A pit between a bast-fibre and a medullary-ray cell 
is also shown. Note the position of the lamella and the character of the pits, x 1,000. 
Fig. 54. A compound sieve-plate in the winter condition in longitudinal section. The large 
callus-cushions are traversed by dotted lines, which are the remains of the slime-strings. Material 
preserved April 4, 1901. x 750. 
Fig* 55 * A sieve-plate in section, showing the recommencement of activity. Slime is beginning 
to accumulate in the sieve-tube and to bore its way along the old pores in the callus-cushions. 
Various stages are seen in the figure. Material preserved April 22, 1901. x 750. 
Fig. 56. A callus-cushion in transverse section, showing the formation of new slime-strings in 
the old pores of the cushion on the reopening of the sieve-plates in spring. The strings are thicker 
than those seen in Fig. 55. Material preserved April 24, 1901. x 750. 
Fig. 57. A sieve-plate in longitudinal section from material preserved May 9, 1901. The slime- 
strings are completely re-established and the callus is being rapidly dissolved, x 750. 
Fig. 58. A callus-pad, in transverse section, which is undergoing solution. The pores are well 
seen and the whole cushion shows a stratified structure, which is revealed by the swelling due to the 
ferment action, x 700. 
