118 



in all our text-books, with their accumulations of mucilage above 

 or below the sieve-plates, (^' Schlauchkopfe") etc., must therefore 

 be considered as representations of entirely artificial conditions. 



The author simply immerses the entire plant, or a portion of 

 it, (while still in connection with the plant) in hot water ; thereby 

 the contents of the sieve-tubes will be fixed and the changes 

 mentioned almost entirely prevented. 



According to their contents the sieve-tubes are divided into 

 three groups: first, such as contain sap which coagulates when 

 heated, and which is surrounded by a thin parietal plasma sac 

 {^Cuctirbita) ; second, sieve-tubes containing mucilage masses 

 closely in contact with the parietal plasma, and clear sap which 



does not coagulate (Hi 



with starch 



grains, with only little mucilage in the parietal plasma, and with 

 sap which does not coagulate. This differentiation of the con- 

 tents is met with only in active tubes, /. e, in such as have their 

 plates open. 



The sieve-plates of active tubes are lined with a thin layer of 

 callus which, in its turn, is covered with mucilage either entirely 

 or only at the edges of the perforations. The parietal protoplasm 

 most likely lines the sieve-plates and also the short canals piercing 

 the same, so as to effect a connection of the plasmatic layers of 

 the contiguous members of the tube. 



When a plant is injured accumulations of mucilage are formed 

 at the plates by the streaming of the sap through the pores. At 

 the same time the mucilaginous lining of the plates disappears, 

 while the callus increases in thickness. Both the callus plates 

 and the aggregations of mucilage, therefore, are artificial products, 

 for in the living, active sieve-tubes there is only very little callus 

 on the sieve-plates. In the living plant the pores of the sieve- 

 tubes are not filled with mucilage as long the sieve-tube is active, 

 but with coagulable sap in Cuairbita, and probably with watery 

 sap in other plants. 



The obliteration of the cribrose tubes begins with changes of 

 the contents and the plates. In Cucurhita there appear in the 

 sap some drops of mucilage and a coarse coagulum. Then the sap 

 becomes quite rigid, but soon separates into small portions which 

 are again dissolved and removed, or the sap gradually loses its 



f 



