346 



MANUAL OF BOTANY 



In some aquatic roots the perioyole is not present, but the 

 vascular tissue of the stele abuts directly on the endodermis. 

 This is the case also in the roots of the Eguisetacece, and in the 

 stems of Salvinia and Azolla^, where the endodermis is two- 

 layered. 



The Vascular Bundles. — Each fully differentiated vascular 

 bundle consists usually of two parts, called the xylem, or wood, 

 and the phloem, or bast. The xylem is made up of what has 

 been described as tracheal tissue mixed with a certain amount 



Fit;. 731. Collateral bundle of /licinnx. rn. Enrlodermis. ph. Phlob'm. 

 i: Cambmm. px. Protoxylem. .r. Xylera. if.c. IntGrfascicular cam- 

 bium. After Sachp. 



of wood parenchyma and wood fibres. Its walls are always 

 lignified, and usually the cells are empty of contents. The 

 phloem is made up largely of sieve tissue with a little paren- 

 chyma. Associated usually with it is also a certain, sometimes 

 a large, number of fibres. Bast or phloem cells are never 

 lignified, and always contain protoplasm. 



In addition to wood and bast certain bundles contain a 

 thin layer of meristem, known as cambium. 



The wood and bast, though usually combined into a single, 

 or conjoint, bundle, may be separate, when they are spoken of 

 as wood or bast bundles respectively. 



