130 IX. THE DICOTYLEDONOUS STEM HERBACEOUS TYPES. 



member of the sieve-tube, by longitudinal division of a single cell, 

 and that this original companion-cell divides by repeated cross- 

 divisions into a row of superposed companion-cells. They have 

 abundant protoplasm and a large nucleus, and are connected 

 with their sieve-tubes by elongated pores. The companion-cells 

 of the successive segments of a sieve-tube often do not join on to 

 one another. Amongst the sieve-tubes and companion-cells are 

 elongated thin- walled bast parenchyma. 



It is very instructive to treat a longitudinal section of alcohol 

 material with concentrated sulphuric acid. The walls of the sieve- 

 tubes and the sieve-plates are dissolved. The slime-strings of old 

 sieve-tubes, however, remain unaffected, and we can thus obtain 

 preparations which show as in D in Fig. 47, and demonstrate in 

 the clearest manner that the contents of the constituent cells join 

 one another through the pores of the sieve-plate. These prepara- 

 tions can be washed by running water under one edge of the 

 cover-glass, and withdrawing it from the opposite edge by means 

 of blotting-paper ; and can then be stained with a drop of aniline 

 blue. 



In order to obtain a thoroughly accurate idea as to the natural 

 distribution of the contents of sieve-tubes, it is necessary to have 

 recourse to another method of preparation, viz., to fix the con- 

 tents of the sieve-tubes of a pot plant of Cucurbita with boiling 

 water, not, however, previously severing the stem, but bending it 

 down under w r ater for about five minutes. Material thus fixed 

 can be either investigated direct, or can be preserved at will, 

 and without further change, in alcohol. The contents of the 

 sieve-tube will then be fixed in their position as during life, and 

 it will be seen that the Slime-strings and slime-plugs of the 

 alcohol material are due to the method of preparation, and the 

 plug no doubt to the stem having been cut before the action of 

 the reagent. Such material will also show us that in active 

 sieve-tubes the callus forms only a thin layer on the sieve-plate, 

 while it thickens greatly in those which have ceased to function ; 

 similarly, in the former case the sieve-pores are filled, not with 

 mucus, but with coagulated sieve-tube sap, while in the latter 

 case the callus layer almost fills up the pores. 



We may also prepare longitudinal sections of fresh material. 

 In these the sieve-plates show just as clearly as in alcohol 

 material. The accumulations of slime at the sieve-plates can be 

 well seen ; but the slime nowhere shows as a special string with- 



