Hill . — The Histology of the Sieve-Tubes of A ngiosper ms. 251 
who finds that it is pectic in composition, when deprived of its callus, and 
gives no cellulose reactions. 
Such then was the condition of our knowledge of the sieve-tube and 
phloem histology in 1899 when Perrot wrote his memoir, in which he 
accepts nearly all Lecomte’s results. The next contributions of a serious 
character are those of Strasburger. In his important paper on proto- 
plasmic connexions, 1 published in 1901, he still held the view put forward 
in an earlier work 2 that with the formation of the callus-rod from the 
original connecting-threads the complete development of the sieve-plate has 
been reached. In his review 3 of my paper on the sieve-tubes of Pinus f 
he accepts certain of the conclusions there put forward and agrees that it 
is the slime-string and not the callus-rod which plays the important role in 
the communication between two sieve-tubes. 5 
In the paper published in 1901 he also traces the history of the 
development of the sieve-plate and sieve-fields in Wistaria chinensis 
(. Kraunhia ), and other Angiosperms, and, since the results there stated 
differ considerably from those in general acceptance at that date, it will 
be necessary to give some account of them and also to add some 
criticisms. 
According to Strasburger, the young sieve-plate membrane soon after 
its formation becomes covered by a network due to local thickening. The 
whole membrane is then overlaid by a substance which is characterized by 
the readiness with which it stains blue with aniline blue, and at this period 
the fine protoplasmic threads are seen in the sieve-fields. 6 The threads, 
however, soon begin to take up aniline blue, and, in favourable cases, a 
surface view effect can be obtained of a fine network in a sieve-field corre- 
sponding to this perforation of the membrane (v. Fig. 30, PI. XIV). So far 
this agrees with the course of development given by him for Pinus , to 
which reference has already been made. The further stages of the develop- 
ment, however, are different. The aniline blue staining substance which 
overlies the whole * sieve-plate accentuates particularly the height of the 
network, and continuity across the unstained portion of the membrane is 
1 Strasburger, in Jahrb. f. wiss. Bot., Bd. xxxvi. 
2 Strasburger, Leitungsbahnen, 1891, pp. 60 et seq. ; v. Hill, Histology of Sieve-tubes of Pinus , 
Ann. Bot., vol. xv, 1901, pp. 582-3. 
3 Strasburger, in Bot. Zeit., vol. lx, 1902, p. 49. 
4 Hill, Ann. Bot., vol. xv, p. 575. 
5 1 . c., p. 47, Strasburger states that in his ‘ Kleines Practicum,’ fourth edition, he had corrected 
the figure of the sieve-plate of Pinus from the figure given in his paper before my work was published ; 
his figures are certainly somewhat altered, but in both cases they appear to be so badly reproduced 
as to be almost unintelligible. Moreover, in the ‘ Practicum,’ fourth edition, there is no adequate 
explanation of the altered figures in the text ; cf. p. 85, Fig. 49. 
6 Cf. Strasburger, 1 . c., Fig. 29, PI. XIV. The sieve-fields in Strasburger’s sense are the small 
sieve-areas or pits of the single sieve-plate, they thus correspond to Lecomte’s meshes of the sieve- 
plate network. Other writers limit the term ‘ sieve -fields’ to the thread groups on the lateral walls of 
the sieve-tubes ; and it is so used in this present paper. 
S 2 
