Lecture XIX. 151 



the protoplasm of adjoining sieve-tubes through the perforations 

 of the sieve-plates. The central cavity of the sieve-tubes is filled 

 with a dilute sol of proteins often containing also dissolved sugar. 

 Filling up the interstices between the tracheids in the wood and 

 the sieve-tubes in the bast are thin walled polyhedral cells with 

 cellulose walls and protoplasmic, and often starchy contents. Their 

 diameters in different directions being approximately equal, they 

 are called respectively the wood and bast-parenchyma. The outer- 



FIG. 35. Aspidium filix-mas, small conducting tract or vascular bundle, 

 longitudinal section, x 170. a, tracheae ; b, bast ; c, bundle sheath. 



most layer of the conducting tract is formed by the endodermis. It 

 is easily recognised by its being next the brown cell-walls of the sheath 

 isolating the strand from the surrounding fundamental tissue. The 

 cells of the endodermis are oblong. Their greatest length is parallel 

 to the axis of the bundle and they fit together closely, leaving no 

 gaps. Between the endodermis and the outside of the bast is a 

 single layer of cells, the pericycle. The cells of the pericycle 

 roughly correspond with those of the endodermis. The walls 

 of the endodermis and pericycle are uncoloured. 



