5 76 
Hill. — The Histology of the 
Historical. 
The first mention of the sieve-tubes of Gymnosperms was 
made by De Bary 1 in 1877, who described and figured those 
of Encephalartos and Sequoia. His observations dealt es- 
pecially with surface-views, in which he noticed the net-like 
structure of the rounded sieve-areas, and also saw a delicate 
sieve-structure in the meshes of the sieve. His figures agree 
closely with Strasburger’s surface-view figures, but he failed 
to notice a development of callus. 
The papers, however, which especially deserve attention are 
those by Janczewski, Russow, Kienitz-Gerloff, and Stras- 
burger. 
Janczewski 2 in 1881 gave a detailed account of the de- 
velopment of the sieve-plate. He found that the developing 
sieve-plate is composed of more or less refringent particles 
alternating with each other. In a more advanced stage the 
more refringent particles, which are also the larger, are 
found to have become altered into callus at their free surfaces, 
whilst in the middle they still retain their primitive character, 
so that the closing membrane shows paired knobs of callus 
arranged on either side of it. 
The alteration of the refringent particles proceeds rapidly 
until a cylinder of callus is formed, still showing traces of 
the original unaltered membrane at its centre ; but this too is 
quickly replaced and a solid callus-cylinder or rod is formed. 
After this the heads of these cylinders or rods swell up and 
fuse around the cellulose network (composed of the less 
refringent particles of the primitive wall), and then finally the 
callus is completely dissolved, leaving open pores between 
neighbouring sieve-tubes. Thus Janczewski only saw rods of 
callus traversing the pit-closing membrane, and was not able 
to observe anything in the nature of protoplasmic connexion, 
or of the finer sieve-structure figured by De Bary. 
1 De Bary, Comp. Anat. (Eng. ed.), p. 179, Fig. 77. 
2 Janczewski, Mem. de la Soc. des Sc. Nat. de Cherbourg, 1881. Ann. des Sc. 
Nat., Bot., 6® ser., tome xiv, 1882. 
