586 Hums . — The Histology of the Sieve Tubes of Pteridium 
traverse the lateral sieve plates. A certain amount of granular protoplasm lines the sieve tube and 
is aggregated at the sieve plate, x 650. 
Fig. 2. A group of protophloem elements in longitudinal section. The contents are compara- 
tively dense, especially in the pits. The middle lamella is not visible, and only one of the protoplasmic 
threads shows a median node, x 650. 
Fig. 3. Longitudinal section of two protophloem elements connected by protoplasmic threads. 
Middle lamella and median nodes are faintly visible, x 730. 
Fig. 4. Transverse section of two adjoining sieve tubes. Aggregation of the granules is more 
marked and callus formation has commenced on both sides of the plate, but has advanced somewhat 
further on one side than on the other. Alteration of the middle lamella to form the median nodule 
has begun, and the staining of the threads is darker, x 600. 
Fig. 5. Transverse section. A very common aspect of stages containing callus, in which only 
the median nodule and the paired callus basins can be seen. The layer of protoplasm covering the 
plate is thicker but less granular than usual, x 1,000. 
Fig. 6. Transverse section, showing groups of paired callus basins with their median nodules in 
the same stage as Fig. 5. The large empty lumen and parietal layer of protoplasm of the sieve 
tubes and the dense contents of the abutting phloem parenchyma cells are shown, x 550. 
Fig. 7. Longitudinal section, showing lateral sieve plates of radial wall in surface view. 
Stained with London blue only so that the callus basins alone are shown as blue dots. (Semi- 
diagrammatic.) 
Fig. 8. Longitudinal section, showing a much inclined terminal wall in suiface view. Cf. 
Fig. 7 ; the sieve plates are larger and much more numerous than in a radial wall. Stained with 
London blue only. (Semi-diagrammatic.) 
Fig. 9. Longitudinal section of a sieve tube wall bearing numerous sieve plates. Callus basins 
and median nodules have been developed and the protoplasmic threads are undergoing conversion 
into the more darkly staining slime strings. Granules in some cases show attachment to slime 
strings, x 650. 
Fig. 10. Longitudinal section, showing a much inclined terminal wall in surface view. An 
exceptional case in which there is a considerable development of callus over the surface of the 
plate. The granules show a regular arrangement in relation to the ends of the slime strings, x 700. 
Fig. 11. Transverse section of group of abutting sieve tubes, showing accumulations of callus 
on two lateral sieve plates, x 480. 
Fig. 12. Longitudinal section of a lateral wall with numerous sieve plates; a slightly more 
advanced stage than Fig. 9. The slime strings are thicker, and the relation of the granules to the 
slime strings is more definite. Solution of the callus has commenced, and its outline has become less 
sharp, x 650. 
Fig. 13. Surface view of a large inclined terminal plate. The sieve plates are very numerous 
and show the distribution of the refringent granules all over them, each granule concealing a slime 
string beneath it. x 750. 
Fig. 14. Surface view of radial wall. The sieve plates are smaller and much less numerous. 
Refringent granules as in Fig. 13. x 600. 
Fig. 15. Surface view of two sieve plates in optical section. The refringent granules conceal 
the underlying callus and slime strings, except in one or two cases where the granule has been 
dislodged and the callus has become visible. The median nodule is focused as a dark dot in the 
midst of the callus basins and is also visible through one or two of the granules, x 800. 
Fig. 16. Two sieve plates in surface view. The granules have increased in size. One granule 
dislodged, showing corresponding underlying callus basin, x 800. 
Fig. 17. Longitudinal section of a stage corresponding to surface view in Fig. 16. Paired 
callus basins, median nodules, and refringent granules, some of which have been dislodged, 
x 800. 
Fig. 18. Transverse section of adjoining sieve tubes with lateral plate between them. Three 
slime strings, considerably thickened, with granules attached, cross the sieve plate. The callus is 
partially dissolved away, leaving only a thin tubule ; the median nodule is still present, x 600. 
Fig. 19. Longitudinal section of single sieve plate. Callus, with slime strings not much 
thickened, but with granules attached on one side of the plate. Callus dissolved away on the other 
side, one granule sinking in and another detached not far off. x 800. 
Fig. 20. Longitudinal section of three slime strings, the top one cut obliquely ; the callus is 
