122 THE ELEMENTS OF VEGETABLE HISTOLOGY 



in the spiral and annular types is such as to afford 

 the maximum of support with the minimum of material, 

 and these forms are usually present in leaves or other 

 parts of the plant which do not survive more than 

 a season. 



Tracheids are conducting cells in which the walls 

 between adjacent cells have not been entirely obliter- 

 ated. Communication between the various cells form- 

 ing a tracheid is effected by means of pores in the 

 vessel walls. The pits in the tracheid walls are termed 

 bordered pores and each appears as a small elevation 

 in the center of which may be a valve-like flap of 

 tissue. 



In general the ducts conduct materials toward 

 the leaves. The materials contained within the duct 

 are simple substances in aqueous solution and are to 

 be transformed by the leaves into nutrients. Vari- 

 ous types of ducts are illustrated in Plate 45. 



Sieve Tubes. — Sieve tubes differ from ducts in 

 that they consist of single cells, each of which is a 

 unit in the transportation of materials. The sieve 

 tubes are formed by perforations in the end walls of 

 each cell of a vertical row. These perforated end 

 walls between adjacent cells may be more or less 

 consolidated so that the pores of each wall coincide 

 with those in the walls of the cells above and below. 

 The perforated and partially consolidated end walls 

 are termed sieve plates. In transverse sections of cells 

 containing sieve plates the latter appear as yellowish 

 perforated structures entirely filling the cell (Plate 

 46, No. 2). Upon longitudinal section the true nature 

 of sieve plates is apparent and they are found to be 

 yellowish perforated partitions between cells of a verti- 



