Hill.— The Histology of the Sieve-Tubes of A ngio sperms. 24 7 
views in connexion with his researches on the continuity of protoplasm , 1 
and also, no doubt, to the absence of figures, his conclusions have not 
received the recognition which they deserve. Though his methods were of 
a very drastic nature it is remarkable that they have yielded results which 
have been proved to be substantially correct by our modern histological 
technique. He distinguishes clearly between the callus and the slime- 
strings which perforate it, not only in the large sieve-plates but also in the 
sieve-fields, and shows that the strings in the canals of the sieve-tubes are 
smooth and homogeneous throughout. He considers therefore that they 
cannot be of a protoplasmic nature, though in his earlier paper these strings 
were regarded as being of the nature of callus, owing to the brown colora- 
tion they gave with his callus reagent. In sieves which function for more 
than one year Russow realized the important part played by such strings (or 
Stifle ) in the reopening of the sieve-plate after the winter’s rest, though he does 
not seem to have understood either the condition of the callus-plate during 
the dormant period or the method by which it again becomes functional . 2 
With regard to other connexions in the phloem Russow could get no 
proof ; he was unable to see any threads between the sieve-tubes and 
companion cells, or between these latter and the bast parenchyma or the 
medullary ray cells. No doubt his failure to obtain results in these cases 
was largely due to the disorganization of the cell-walls by the reagent 
employed . 3 His views on the early history of the sieve- plate differ from 
those of Wilhelm and Janczewski. These observers held the opinion that 
the callus arose in little spheres in pairs on either side of the young sieve- 
plate, owing to the transformation of the cellulose. These then fused 
together to form the stoppers of callus filling the pores of the sieve-plate, 
and finally the stoppers are pierced by the connecting filaments. 
Russow, on the other hand, was of the opinion that the young sieve- 
plate is first pitted with corresponding pits on either side of the membrane ; 
the callus is then formed in these little depressions by the protoplasm and 
not by a transformation of the cellulose . 4 The callus appears in the form of 
little basins, with a polygonal outline, and Russow attaches considerable 
importance to its shape as proof that the callus substance is formed by the 
contents of the sieve-tube and deposited on the surface of the sieve ; the 
gradual overlaying of the cellulose sieve and the great size to which the 
callus may subsequently attain afford further proof, he considers, of the 
correctness of these views. 
1 Russow, 1 . c. 1883, p. 570. 
2 According to Janczewski, 1 . c., p. 145, Figs. 10-12, PI. VIII, the sieve is entirely shut by 
a homogeneous callus mass in Vitis, and is reopened with the formation of striae about mid- April. 
3 Russow used about 81 per cent, sulphuric acid. 
4 Gardiner, £ On the constitution of the cell-wall and middle lamella in Proc. Camb. Phil. Soc., 
vol. v, p. 15, ibid., ‘ Observations on the constitution of callus’, p. 230, agrees with the views put forward 
by Russow as to the protoplasmic derivation of the callus, and considers this substance to be of the 
nature of a starchy mucilage. 
