248 Hill . — 77 ^ Histology of the Sieve- T ubes of A ngio sperms. 
Neither the solution of the cellulose lamella between the two opposed 
callus-basins nor the formation of the connecting filaments was observed. 
Both Janczewski and Russow have investigated the sieve-tubes of 
Gymnosperms also, but their results need not detain us here, since they 
have been discussed elsewhere. 1 The papers of Fischer, 2 which follow in 
chronological order, deal with the physiological rather than with the 
histological aspect of our subject, one of his objects being to show that the 
ordinary contracted appearance of the sieve-tube contents and the charac- 
teristic masses of slime ( Schleimkopf ) seen on one side of the sieve-plate 
have been artificially produced, and that in the normal condition the sieve- 
tube contents are homogeneous. 
One of his papers is of some histological interest, since he employed 
Russow’ s method, and two of his figures probably show the sort of results 
which Russow must have obtained. In both cases the results are artificial 
and untrue, the cell-walls being enormously swollen. In the one case the 
wall between two sieve-tubes shows stratification with 1-4 threads in each 
pit, and, in the other, direct continuity by single hair threads 3 is figured 
between a companion cell and a sieve-tube. 
In the summary of his results he states that the sieve-tubes are in 
‘ direct continuity’ with one another through fine threads and with the com- 
panion cells, 4 and that the cambiform cells are in continuity with one another, 
but not with either the sieve-tubes or the companion cells. Fischer’s results 
seen by the light of his figures are so misleading that they cannot be con- 
sidered as a very valuable contribution to the histology of the phloem tissues. 
The next work on this subject is Lecomte’s careful study of the phloem of 
Angiosperms 5 , a good deal of which is occupied with questions of histology. 
His own work is collected into nine chapters, of which his chapters IV, 
on the Development of the Sieves, 6 and V, on the Development, Form, 
Structure, and principal reactions of the Callus. 7 concern us more particularly 
in this paper. 
Lecomte, after a useful review of previous work, proceeds to give the 
results of his own researches, which in some points differ fundamentally 
from those to which allusion has already been made. According to him, 8 
the young thin membrane— the future sieve-plate — is at first composed of 
some nitrogenous body ; this soon becomes coated with cellulose in such a 
way that the cellulose forms intersecting bands, leaving the meshes of the 
1 Hill, A. W., Histology of the sieve-tubes in Pinus, in Ann. Bot., 1901, vol. xv, pp. 576-80. 
2 Fischer, v. list of Literature cited at end. 
8 There is no doubt that the so-called threads are the pit fillings of paired pits so drawn out by 
the intense swelling of the wall that they appear like a fine continuous thread. 
4 Fischer, A., Neue Beitrage zur Kenntniss der Siebrohren. Ber. fiber Verhandl. der kon. Sachs. 
Ges. d. Wissen. zu Leipzig, 1886, p. 297. 
5 Lecomte, in Ann. des Sq. Nat., Bot., Ser. vii, 1889, vol. x, p. 193. 
6 Ibid., p. 244. 7 Ibid., p. 258, 
8 Ibid., pp. 248 and 250, Fig. 9, PI. XXI, Fig. 12, PI. XXII. 
