SPECIAL METHODS. 249 



bita and many other plants when the ordinary methods are 

 employed, the so-called " Schlauchkopfe " (Fig. 63, a, s), 

 represent artificial products and originate only when the 

 sieve-tube system is cut. They were entirely wanting in 

 material scalded as above (Fig. 63, b), but were not changed 

 by boiling water in tissues where they had arisen from cut- 

 ting before the scalding. 



456. According to investigations of this scalded material 

 by A. Fischer (VI), we may distinguish three sorts of sieve- 

 tubes, according to the organization of their contents, as 

 follows : 



1. Sieve-tubes with coagulable contents (in the Cucurbi- 

 tacea) have a thin protoplasmic wall-layer and a clear sap 

 coagulable by heat. 



2. Sieve-tubes with mucus (as in Humulus] contain a 

 delicate wall-layer filled with large and small slime-masses 

 and a clear, non-coagulable, watery fluid. 



3. Sieve-tubes with starch-grains (as in Coleus] contain a 

 delicate wall-layer carrying small masses of mucus and 

 a clear, non-coagulable fluid with small starch-grains. 



