SIEVE-TUBES OF PJNUS. 151 



are tilled with air, and on that account appear dark and are 

 easier to see. In alcohol material the alcohol has removed 

 the air. 



A section of the bast which will answer our requirements 

 cannot be obtained without trouble. There is no recourse but to 

 commence with the older and outer part of the bast, and prepare 

 a considerable number of successive sections, till we have reached 

 the young wood. We scan these sections with a low power 

 objective, and look for such as contain still active sieve-tubes, 

 By this means we can learn something about the callus-plate. 

 which at once attracts the eye, even without staining and with slight 

 magnification, as a strongly refractive pad (Fig. 57, fi] attached to 

 the cell-wall. The sections of the sieve- 

 plates can be best studied in chlorzinc 

 iodine, to w T hich we have added an equal 

 bulk of dilute watery potassium - iodide 

 iodine solution. The iigure of the sieve- 

 plate in' this view is the same as in the 

 cross-section, but the number which are 

 cut is very considerable, and therefore it is 

 more easy to obtain a favourable view. 

 They will be found most quickly at the 

 edges of the section. The sieve-plates (Fig. 

 57, A] are seen in profile, in the radial \valls 

 of the sieve-tube which we have cut through 



with the razor. The walls themselves are 



FIG. vl. 



somewhat swollen in the chlorzinc iodine Parts of the walls of sieve- 



solution, and have taken a violet colour. 



The sieve-plate is, so long as it appertains the formation of the callus- 



plate; B, after its formation: 

 to a still active sieve- tube, stained reddish- c', from a sieve-tube which 



brown. This colour proceeds from the ^it^x 540)" ^ 

 strings of plasma which penetrate from 



both sides into the sieve-pore. It appears then as if the sieve-plate 

 were pierced by reddish-brown rods (compare the figure). The 

 callus-plates (B) have coloured reddish-brown, provided the chlor- 

 zinc iodine solution is not too concentrated, and has thereby acted 

 as a solvent. The sieve-plates of sieve-tubes which have lost 

 their function (C) appear bright violet ; the strings of plasma and 

 the callus-plates have disappeared from these. If we stain such 

 a tangential longitudinal section with aniline blue, and examine 

 it in glycerine, the brilliant blue callus-plates at once attract 



