587 
Sieve-Tubes of Finns . 
cell-walls and the youngest layer of these active tubes, which 
was designated as the ‘ boundary-cell layer ’ by Russow, 
appears as a well-marked line limiting the zone of the active 
sieve- tubes (Fig. i, PL XXXII). 
These thick walls of the active sieve-tubes, which have been 
examined by Leger 1 - — whose experiments I have repeated — 
are found to be complex in structure. If the sieve-tube walls 
are acted upon with acid or basic dyes, such as Congo red or 
methylene blue, which stain cellulose and pectic substances 
respectively, it is found that the cell-walls of the young 
developing sieve-tubes are composed of pecto-cellulose like 
those of ordinary parenchymatous cells ; soon, however, they 
become covered internally by a layer of pure cellulose, so that 
in a mature sieve-tube, as seen in a transverse section, there is 
a middle lamella of pecto-cellulose with a broad band of pure 
cellulose on each side. The pit-closing membranes of the 
active sieve-tubes show a similar structure (Figs. 2 , and 3, 
PL XXXII). The structure of the mature sieve-plate, as will 
be pointed out later on, has an important relation to the 
complex nature of the cell- wall. A better idea of the changes 
which take place in the developing sieve-plates will, perhaps, be 
obtained if the structure of the mature sieve-plate is first 
described in detail. The pit-closing membranes or sieve- 
plates, which are somewhat lenticular in cross-section, are seen 
to have a circular or oval outline in surface-view with small 
angular or rounded areas, the sieve-fields scattered over the 
surface ; in a section of the sieve-plate these fields are found 
to correspond to the callus-rods (Figs. 2,1 and 24, PL XXXIII). 
Enclosed in the callus-rods are the slime-strings, some five 
to nine in number, as Russow describes ; and at the middle 
lamella there may be seen either a single highly refractive 
median-nodule, dividing the callus-rod into two parts, or each 
slime-string may be seen to possess a median node — a result 
depending on the treatment the section has undergone. It 
thus appears that at the middle lamella there is a large 
refractive nodule, which is a part of the middle lamella, 
1 Leger, M^m. de la Soc. Linn, de Normandie, 1898, tome xix. 
