86 PEACTICAL BOTANY. 



The callus mass swells : the protoplasm also swells : 

 the section thus becomes more transparent, and the 

 cellulose basis or true sieve becomes more apparent, 

 and its pores can be easily seen. For further reactions 

 of the callus, see p. 31. 



IV. Treat some fresh sections with iodine, then dry 

 off the superfluous fluid with blotting-paper, and 

 mount in a single drop of strong sulphuric acid. The 

 cellulose walls and callus will swell; the protoplasm 

 will contract; look carefully over the protoplasmic 

 contents of the sieve-tubes for the points where sieve- 

 plates have been ; here it will be found that fine strings 

 of protoplasm, which passed through the sieve -plate, 

 connect the protoplasmic masses on opposite sides of 

 the sieve with one another (cf. Sachs' Textbook, Fig. 47.) 

 By this reaction the continuity of protoplasm through 

 the sieve is demonstrated. 



It will be noted that the sieve-tubes of Cucurbita, 

 closely resemble those of Helianthus, the sieve-plates 

 being transverse and simple. This is the usual type of 

 sieve-tube to be found in primary phloem of Angio- 

 sperms, and generally in herbaceous stems of the same 

 group. In the secondary phloem of ligneous stems a 

 more complicated type of sieve-tube is found. This will 

 be studied below 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. Mount in glycerine 

 and examine with a high power for sieve-tubes. The 

 general arrangement of the phloem is similar to that in 



