128 IX. THE DICOTYLEDONOUS STEM HERBACEOUS TYPES. 



bitacese the end sieve-plates are placed horizontally ; hence in 

 both portions of the bast they will probably be exposed in full- 

 face view. They are readily recognisable from their areolation 

 (Fig. 47, A) and high refraction. The small companion-cells (s) 

 will also be recognisable from the density of their plasmic con- 

 tents. If a section is treated with soda corallin, the sieve-plates 

 show up very clearly with a rosy colour ; if treated with aniline 

 blue they are deeply stained by it. 



Radial longitudinal sections, which have cut through a 

 vascular bundle, show the tracheides and vessels with their 



A 



FIG. 47. Cncurbita Pepo. Parts of sieve-tubes. A, in cross- section, B to L), in 

 longitudinal section. A, a sieve-plate seen in full face; B and C, side-view of the 

 adjoining parts of two sieve-tubes ; D, the connected parts of the slime-strings of two- 

 sieve-tubes, after the action of sulphuric acid ; s, companion-cells ; u, string of slime ; 

 pr, protoplasmic lining layer ; c, callus-plate ; c*, small unilateral callus-plate from 

 a lateral sieve-plate (x 540). 



characteristic thickening, and that the two big ducts are con- 

 structed of very short barrel- shaped segments. Their pits are 

 bordered ; those of the xylem parenchyma are unbordered 

 (simple). 



Structure of the Sieve-tubes. Longitudinal sections of alcohol 

 material will enable us to study the unusually large sieve-tubes. 

 They are long continuous sacs, the successive segments of which 

 are separated by horizontal sieve-plates (Fig. 47, B). Study 

 will be facilitated if we lay the longitudinal section for a short 

 time in aniline blue, and afterwards examine in glycerine. After 



