CONDUCTING TISSUES 121 



row of adjacent cells and subsequent absorption of 

 the end walls of these cells. The walls of ducts are 

 partially lignified and therefore consist of ligno- 

 cellulose. As the duct is a structure formed from 

 many cells, each of which has lost its identity, proto- 

 plasmic contents are absent and the tissue is prac- 

 tically lifeless. Differences in the arrangement of 

 the lignified substance within the vessel offer a con- 

 venient basis for classification. Lignification takes 

 place in a systematic manner, each species showing 

 peculiarities which may often be used as a factor in the 

 identification of a given plant. Different organs of 

 a plant will show different types of vessels, although 

 the type is definite for each organ. The types of 

 ducts include pitted, reticulate, scalariform, annular 

 and spiral forms. Pitted ducts are characterized by 

 the presence of numerous pores or openings through 

 the ligno-cellulose wall of the vessel. The layer of 

 lignin lines all portions of the vessel but shows numer- 

 ous perforations or pores. In reticulate ducts the ligni- 

 fied tissue is in the form of a network upon the inner 

 face of the vessel wall, and the portions not covered 

 by lignin are irregular in outline. The non-lignified 

 portions of the walls of scalariform vessels are in the 

 form of long narrow slits showing fair uniformity both 

 as regards size and arrangement. Scalariform vessels 

 are often angled, and in this respect differ from all 

 other types. Annular vessels are rather thin-walled 

 tubes possessing rings or hoops of lignified tissue 

 within the lumen of the tube. In the spiral vessels 

 the lignified tissue is arranged in the form of a continu- 

 ous spiral band or collection of bands extending 

 throughout the length of the vessel. Lignification 



