250 Hill — The Histology of the Sieve-Tubes of Angiosper ms. 
substance of the meshes must thus be transformed first into callus and 
afterwards progressively, in certain cases, into an albuminous substance. 
Lecomte’s results, which seem to have been generally accepted 
(certainly by French botanists), not only make the early history of the 
sieve-plate more complicated than really appears to be the case, but also 
include several erroneous conceptions as to the state of the active sieve-tubes 
in Gymnosperms and some Angiosperms. There seems every reason to 
suppose that the sieve-tubes in these two groups of plants are — at any rate 
in the active condition — always in communication with one another by 
means of mucilaginous slime-strings . 1 Further, the callus is always visible 
before any perforation of the membrane by the slime-string has taken 
place, as Russow and Wilhelm observed , 2 and Lecomte does not appear to 
have noticed the little paired basin-shaped patches of callus separated from 
each other by a thin intervening piece of the membrane, which were 
accurately described by Russow and form so marked a feature of the 
developing sieve-plate. Nor does he allude to the callus lining the holes of 
the sieve-plate, which has been well figured by Wilhelm . 3 According to 
Lecomte the callus first arises as a transformation of the substance covering 
the cellulose network of the sieve, whilst according to Wilhelm, Janczewski, 
and Russow, with whom on this point the present researches are in accord, 
the callus makes its appearance in the meshes, depressions, or pits of the 
young sieve-plate, i. e., the spaces between Lecomte’s hypothetical inter- 
secting bands of cellulose. His theory of the origin of the perforations in 
the sieve-plate and of the value of the substance of the meshes as the 
pathway for diffusion will be referred to again later. It is enough to add 
here that the knowledge gained by recent histological work on the cell- 
walls and protoplasmic connexions makes this theory highly improbable. 
There are other points of which the explanations are by no means clear, 
such as the nature and function of the striae in old sieves and the way in 
which sieves may be reopened after the winter’s rest. In concluding this 
criticism of Lecomte’s work it must be remarked that his figures are not 
sufficiently clear in many cases to enable one fully to understand his 
meaning . 4 The question of ‘ connecting-threads ’ in the phloem tissues 
does not appear to have engaged his attention. 
A contribution to this subject of the sieve-plate was made by Leger , 5 
No. 16, p. 320). ‘ Le cal est du au d6veloppement exagere de la mince couche de la membrane qui 
recouvre les filaments de cellulose.’ 
1 Hill, Ann. Bot., vol. xv, p. 590, & c. 
2 "Wilhelm, Figs. 12, 14, 15, PI. II; Fig. 25, PI. Ill; Figs. 114, &c., PI. IX. 
3 Ibid. 
4 e. g., his figures of sieve-plates in section and surface view, Figs. 9 and 10, PI. XXI, and 
Figs. 12, 21, and 22, PI. XXII, do not correspond in an intelligible manner, and they are even less 
clear when copied by Perrot, 1 . c., p. 37, and the sieve-plates of Figs. 5 and 6, PI. XXI, and 
F'igs. T4 and 17, PI. XXII, do not appear to represent real structure. 
5 Leger, Mem. de la Soc. Linn, de Normandie, tome xix, 1897, p. 68. 
