5 86 
Hill . — The Histology of the 
whether by the use of Gardiner’s methods any fresh light could 
be thrown on the development and structure of the sieve-plate. 
Most of the work has been carried out in Mr. Gardiner’s 
laboratory, and I must take this opportunity of acknowledging 
the help he has always given me by his invaluable advice and 
kind encouragement. 
The material used in this research was obtained principally 
from the main trunk 1 of a tree of Pinus excelsa , about thirty 
years old, during June, July, and August, and was preserved 
in very small pieces by methods based on those already 
published by Gardiner 2 . Staining was effected by the 
safranin method chiefly, and also by means of the acid-violet 
mixture, and a good contrast staining was produced by 
placing the safranin-stained sections in water blue, the slime- 
strings being stained red and the callus blue. The results 
obtained from an examination of the phloem of Pinus excelsa 
have been compared and found to agree with those from other 
species of Pinus , so that the following account may be taken 
as applying to the genus rather than to any species of Pinus 
in particular. 
Transverse and radial sections of the phloem have chiefly 
been studied, which are of more value than sections cut tan- 
gentially, since the sieve-tubes are seen in chronological order, 
and the various stages in their history from their formation by 
the cambium to their ultimate disuse can be easily traced. 
Before the more minute structure of the sieve-plate is 
considered, a few facts concerning the relation of the sieve- 
tubes to the cambium and the constitution of the cell-walls 
must first be noticed. The cambium is not a very clearly 
defined layer of tissue, since it passes — in the summer con- 
dition — insensibly into the xylem on the one side and into the 
phloem on the other. Between the developing and adult 
sieve-tubes, however, there is a sharp distinction, for the 
mature sieve-tubes possess peculiarly thickened glistening 
1 Russow found that such material gave the best results owing to the large size 
of the cells, and the numerous developmental stages present. Sitzber. d. Dorpater 
Nat.-Ges., 1881, p. no ; 1882, p. 259. 
3 Gardiner, Proc. Camb. Phil. Soc., 1898, p. 508. 
