60 



THE PLANT CELL. 



examination. But in careful preparations made from material 

 fixed in a special manner, it will be found that the protoplasm 

 of each sieve-tube really lines the inner surface of the wall as a 

 thin peripheral layer, in which lie the nucleus and drops of 

 mucilage and food-granules, the central 

 space being occupied by a large vacuole 

 filled with cell-sap. 



The companion - cell is filled with 

 granular protoplasm, and small "pits" 

 in the adjacent walls of tube and com- 

 panion-cell put the protoplasts of the two 

 elements into communication with one 

 another. 



If iodine solution be added to a fre.sh 

 longitudinal section, certain granules in 

 jc the cytoplasm near the sieve-plates turn 

 brown, a reaction which points to the 

 presence of proteid. Globules of mucilage 

 are also to be seen in the mass of con- 

 tracted protoplasm near the sieve-plate. 



Towards autumn, a mass of a substance 

 known as callose is formed on either 

 side of each sieve-area, the whole com- 

 pleted mass being the callus. It stains 

 yellow if treated with solution of aniline 

 sulphate, and bright red with eosin. The 

 callus is deposited by the agency of the 

 cytoplasm, and functions as an effectual 

 plug, which stops up the perforations in 

 the sieve-plate. In the spring of the 

 following year the callus becomes ab- 

 sorbed, and the sieve-tube becomes once 

 r;oMPLETE more functional, but after two or three 

 years a given sieve-tube becomes obli- 

 terated, others having been formed in 



Fig. 41.— A 

 Sieve-tube from Cu- 

 curbitaSTEM. — ji, Cyto- 

 plasm (contracted) ; xp, 

 sieve-plate ; c, callus ; the meantime. 

 X, companion-cell. 



Closely connected with the phloem is a 



tissue which occurs typically in the leaves 



of some plants, notably the centric leaf of Piuus. This, tissue 



is known as transftision - tissue, and its component qells are 



