246 Hill . — The Histology of the Sieve- Tubes of A ngio sperms. 
paper by Strasburger on protoplasmic connexions, published in 1901 ; as 
this will be discussed critically in subsequent pages no account of his 
views need be given here. In the same year a paper on the sieve-tubes of 
Pinus appeared, by the author, which is intended to throw light not only on 
the phloem of Gymnosperms but also on the histology of sieve-tubes in 
general. This was followed by a preliminary note on the sieve-tubes of 
certain Angiosperms in 1903, of which the present long-delayed paper gives 
a more complete account. 
In spite of Perrot’s historical summary, it will help to a better 
understanding of the present position of sieve-tube research if the principal 
results of the various authorities are briefly stated and criticized. In 
particular the points of difference will be indicated in order that the nature 
and scope of the problems still outstanding may be clearly realized. Some 
of these it is hoped may have been solved by the present research. 
Wilhelm worked especially with Vitis and to a smaller extent with 
Cucurbita , and his principal results from the histological point of view 
relate to the sieves and callus. With regard to the companion cells and the 
sieve-tubes, however, he has pointed out their origin from a common 
mother-cell, and he draws attention to the granular protoplasmic contents of 
the companion cells and to their nuclei. He noticed also the numerous 
corresponding pits in the cell-walls between these cells and their sieve-tubes 
and the almost complete absence of pits in the walls between companion 
and cambiform cells. The callus is considered to be formed from the 
cellulose of the sieve-plate, and its origin in the pits of the young sieve- 
plates and the subsequent perforation of these pits by the sieve-pores are 
described. In addition to this he shows that the development and character 
of the sieve-fields on the long walls of the sieve-tubes agree exactly with the 
sieve-plates of the cross walls. The reopening of the sieve-plate in Vitis 
after the winter rest is described with some figures, though the way in 
which this condition is brought about does not appear to have been properly 
understood. 1 The callus of old sieve-tubes which have ceased to function 
as such is considered by Wilhelm to serve as reserve material. The short 
sieve-tubes which cross the medullary rays and link up adjoining groups of 
sieve-tubes are also accurately described and figured, but very little informa- 
tion is given about the development of either sieve-plates or sieve-fields, 
though the early appearance of the callus was noticed. 
This latter question is attacked more particularly by Janczewski, but 
his researches have not thrown very much light on the subject. It is rather 
to Russow, 2 whose work on the protoplasmic connexions was of so 
interesting a nature, that we are indebted for much valuable information on 
the structure of the sieves. Owing to the modification of some of his former 
1 Wilhelm, 1 . c., p. 37, Figs. 89 and 90, PI. VII. 
2 Russow, in Sitzber. der Dorpater Nat. Ges., 1882 ; Trans, in Ann. des Sc. Nat., 1882. 
