Hill . — The Histology of the Sieve-Tubes of A ngio sperms. 287 
Fig. 3. A similar plate to Fig. 2 in surface view ; the protoplasmic processes filling the pits show 
traces of the papillae, x 1,000. 
Fig. 4. Surface view of part of a young sieve-plate showing a group of holes in each callus- 
area. In many cases remains of protoplasmic or slime strings are seen in the holes, x 1,000. 
Fig. 5. A slightly older plate, in which the central part of the callus-area appears to be 
undergoing solution to form the hole for the single slime-string, x 1,000. 
Fig. 6. A young sieve-plate in section, with groups of slime-strings surrounded by callus. The 
original cellulose network of the plate is seen, x 1 ,000. 
Fig. 7. A similar plate to Fig. 6, in surface view. The slime has been removed, and empty 
holes are seen in each callus-area through which the slime-strings passed ; one of the plates shows 
the formation of a single pore from the original group. The cellulose network is deeply shaded, 
x 1,000. (Cf. Text -fig. 8, p. 271.) 
Fig. 8. A sieve-plate showing the formation of single large slime-strings. On the left a large 
pore has been formed. In the other cases the small strings are on the point of being broken down. 
The callus extends across the pit-closing membrane, x 1,000. 
Fig. 9. Portion of a sieve-plate, which has just been opened, in surface view. The pores have 
a narrow lining of callus which also spreads all over the free surface of the cellulose meshwork. 
x 1,000. (Cf. Figs. 4 and 8.) 
Fig. 10. An old sieve-plate in section. The callus has increased, and is blocking the sieves by 
narrowing the pores; the cellulose meshwork is seen at the middle lamella, x 750. 
Fig. 11. An old sieve-plate with callus-pad. The callus stains very faintly with water blue, 
and the pores are marked by dark dots— the remains of the strings. On the lateral wall are two 
sieve-fields with callus-cushions traversed by slime-string canals, x 750. 
Fig. 12. Young sieve-fields in transverse section ; the paired callus-rods reach as far as the middle 
lamella, but do not cross it to come into actual contact with one another. Stained with Russow’s 
callus-reagent, x 1,400. 
Fig. 13. A young lateral wall over-swollen to show the callus-rods abutting against, but not 
crossing, the split middle lamella. Stained with methylene blue and water blue, x 1,000. 
Fig. 14. An older lateral wall between two sieve-tubes in longitudinal section, showing three 
sieve-fields and a single string of another ; each callus-rod encloses a slime-string, and there is 
a conspicuous node at the middle lamella. X 750. 
Fig. 15. A similar wall in surface view with five sieve-fields. The fields have only a few 
slime-strings, and each has its own callus-rod ; the fields are in slight pits, x 1,000. 
Fig. 16. A slightly older sieve-field showing the slime-strings with conspicuous median nodes. 
The callus is not indicated, x 1,000. 
Fig. 17. An old lateral wall between two sieve-tubes, partly in surface view and partly in 
optical section. The callus has formed cushions over the fields, and the groups of median nodes are 
shown, x 1,000. 
PLATE XVII (Figs. 18-25). 
Cucurbita maxima. 
Fig. 18. A. transverse section through the phloem of a root, showing two sieve-tubes with sieve- 
fields in the radial and tangential walls, and companion cells with threads to the sieve-tubes and to 
the bast parenchyma cells ; sieve-tube wall overswollen. The callus is not indicated, x 800. 
Fig. 19. A sieve-plate cut obliquely with a considerable accumulation of callus. The slime- 
strings, though thick, have been drawn out to some length, x 750. 
Fig. 20. An older sieve-plate, almost at the close of its active period; the slime-strings are 
fusiform in shape, having been attenuated owing to the deposition of the callus, x 500. 
Fig. 21. An old sieve-plate with a large callus-cushion. The slime-strings have broken down, 
and lines of dots mark their paths through the callus ; only protoplasm is seen in the sieve-tube. 
X 500. 
Fig. 22. An old sieve-plate in optical section. The meshwork of the plate can be seen through 
the overlying callus. The slime-strings have become very small owing to the filling of the pores 
with callus. (Cf. Fig. 9.) x 750. 
Fig. 23. A very young lateral field with protoplasmic threads. Callus is beginning to be 
formed on one side and appears as little globular heads. (Somewhat overswollen.) x 750. 
