116 PRACTICAL BOTANY 



sieve with one another. By this reaction the con- 

 tinuity of protoplasm through the sieve is 

 demonstrated. 



Another method by which similar results may be 

 obtained is to add a small quantity of dry Hoffmann's 

 blue to a little strong sulphuric acid in a watch-glass, 

 and mix well. Treat the sections with this for a short 

 time, wash with water, and mount in glycerine. 



It will be noted that the sieve-tubes of Cucurbita 

 closely resemble those of Belwnthus, the sieve-plates 

 being transverse and simple. This is the usual type of 

 sieve-tube to be found in primary phloem, and gene- 

 rally in herbaceous stems of Angiosperms. In the 

 secondary phloem of ligneous stems a more com- 

 plicated type of sieve-tube is frequently found. This 

 will be studied now in the stem of the Lime. 



ii. Tilia (Lime). 



I. Cut radial sections of the phloem of a stem of 

 Lime more than three years old. Stain with eosin, 

 wash, and mount in glycerine ; examine them with a 

 high power for sieve-tubes. The general arrangement 

 of the phloem is similar to that in the Elm. The 

 sieves, which will appear stained pink, occur on 

 oblique walls facing the radial plane, and are there- 

 fore here seen in surface view. Note that they have 

 a similar appearance to those above described, but 

 here three or more sieve-plates occur on each oblique 

 wall. 



II. Cut tangential sections of the same ; stain, and 



