SIEVE-PLATES. 



93 



likewise appear upon the lateral walls. When the terminal par- 

 titions are horizontal, or nearly- so, they are cross-plates, the 

 whole partition forming one plate ; but on very oblique ends the 

 plates maj- be separated and lie in one or more rows. The plates on 

 the walls are smaller and irregularly distributed. On parts of the 

 wall contiguous to cells of an3- other kind there 

 may bo dots ; there is j-et some doubt as to 

 whetlier thej- are perforations. 



The diameter of the sieve-pores is given by 

 Mohl as not far from 2 /x, ; but although some 

 are even 5 fx, in diameter, the former figure is 

 too high for the average. 



282. That which 

 is characteristic 

 of sieve-plates, in 

 distinction from 

 groups of perfo- 

 rations elsewhere 

 found, is a thiclr- 

 ening mass, of 

 bluish lustre and 

 apparentlj- homo- 

 geneous struc- 

 ture, known tech- 

 nicall}' as the 

 callus. It is best 

 shown at the ter- 

 minal plates, es- 

 pecially after the 

 application of a 

 solution of iodine 

 which turns it 

 yellow, and makes 

 it more shai-ply defined. In concentrated sulphuric acid and in 

 the strong alkalies this mass swells up so as to be several times 

 its original size ; and in the former it soon dissolves, leaving 

 only slender threads in its place. The character of the callus 



Fig. 74. Pinus sylvestris. Transverse Beution across four entirely passive tubes, 

 wliicli are somewhat compressetl laterally. ^^, (Janczewski.) 



Fig. 75. Pinus sylvestris. Terminal partition. A tube inserted upon the radial 

 wall. Tlie pores of the terminal partition are filled with warty callus, in the midst of 

 which the cellulo.«e network may always be seen; in the pore of the radial wall the 

 callus is completely smooth and round. Tangential section. ^'. (Janczewski.) 



