SIEVE-TUBES 71 



a. A callus-mass which surrounds, and often completely 

 invests the sieve-plate : the size of the callus-mass is variable 

 according to season, age, &c., being greatest in autumn, and 

 in old sieve-tubes. 



b. Protoplasm, which is usually collected in close contact 

 with the sieve-plate (or with the callus if present), and more 

 especially on its upper side. 



Note, i. the oblong form of the segments composing the 

 sieve-tubes. 



ii. The companion-cells, short with granular proto- 

 plasm, and nucleus. 



iii. Cambiform cells of similar form to the segments of 

 the sieve-tubes. 



Other sections should be stained with eosin, then washed and 

 mounted in glycerine. The sieve-tubes will be readily seen, 

 as their contents will be stained deeply. 



III. 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. 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, and generally in herbaceous stems of 

 Angiosperms. In the secondary phloem of ligneous stems a 

 more complicated 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 



