6o8 
Hill . — The Histology of the 
in regulating the size of the pores of the active sieve-plate, 
and that the further development of the callus-cushions 
attenuates and finally closes them altogether. When this 
has occurred the activity of the sieve-tube is at an end, and 
it then, becomes a dead and empty member of the phloem. 
EXPLANATION OF FIGURES IN PLATES 
XXXI, XXXII, AND XXXIII. 
Illustrating Mr. A. W. Hill’s paper on the Sieve-Tubes of Finns. 
PLATE XXXI. 
Diagrammatic Figs, i-io, to illustrate Rus sow's views. 
Fig. t. A sieve-plate in surface- view, showing the sieve-fields (/.) with the sieve- 
pores or canals (sp.). 
Fig. 2. A sieve-plate in optical section; showing the outlines of callus-cushions 
(c.) enclosing the darker cross-sections of the callus-rods (/.). 
Fig. 3. A sieve-plate in section, showing the arrangement of the callus-rods in 
groups, the membrane (w.) between the rods is stained violet with chlor. zinc. iod. 
Some of the callus-rods (g.s.) show dark edges. (‘ Callus-stifte.’) (s.s.)" 
Fig. 4. Diagrammatic view of the complex callus-rods, showing the sieve- 
pores (p.) and canals with the rodlets (?.s.) and the median granules (ml). In 
his 1883 paper Russow shows that the rodlets are slime-strings (cf. Fig. 5, 
PL XXXII). 
Fig. 5. A callus-rod (g.s.) enlarged, showing the two callus-corks or -rods 
(‘ Callus-propfen ’). 
Fig. 6. After treatment with potash the two corks or rods (r.) have separated, 
and appear united by a fine thread. 
Fig. 7. A callus-cushion (c.), showing lines radiating from the rodlets, which 
consist of ‘ Stiftsubstanz ’ (Ss.), the radiating lines are the ‘ Stiftmassen ’ (Sm.). 
Fig. 8. The development of the callus (c.) in the form of refringent particles, 
which arise in the crenulations of the sieve-plate ; traces of the * connecting threads ’ 
( c.t .) are also seen. 
Fig. 9. The formation of the callus-rods (g.s.) ; in some cases the appearance of 
the £ double-brown stripes ’ is seen. 
Fig. 10. The development of the callus-cushion (c.) owing to the swelling up of 
the heads of the rods. 
Di agrammatic Figs. 11-13, illustrating the present research . 
Fig. 11. A mature sieve-plate, showing callus-rods (r.) and nodules (m.n.) at the 
middle lamella (m.l.) enclosing the slime-strings (s.s.) with their median nodes (m.). 
Fig. 12. A sieve between a sieve-tube and an albuminous cell. On the sieve-tube 
