MORPHOLOGY OF HIGHER PLANTS. 191 



The tracheids are intermediate in character between ducts 

 and wood fibers, resembling the former in possessing bordered 

 pores (Fig. 103) and scalar if orm thickenings; and the latter in 

 being true cells, which are usually elongated and quite thick- 

 walled, the walls giving distinct reactions for lignocellulose with 

 phloroglucin or aniline sulphate. 



One of the chief characteristics of tracheids are the bordered 

 PORES (Fig. 103). These differ from simple pores in that the 

 wall surrounding the pore forms a dome-shaped or blister-like 

 protrusion into the cell. On surface view the pores are either 

 circular or elliptical in outline, the dome being circular or, if the 

 pores are numerous and arranged close together, more or less 

 polygonal (Fig. 102, B). 



The number and distribution of bordered pores in the Coni- 

 ferae are quite characteristic for some of the genera, and may be 

 studied in any of the pines, the pores being most numerous in 

 the radial walls. 



The sieve (sieve tubes) is distinguished from the other con- 

 ducting elements in that the walls are thin and are composed of 

 cellulose (Fig. loi, F). It consists of superimposed elongated 

 cells, the transverse walls of which possess numerous pores which 

 are supposed to be in the nature of openings, permitting of the 

 direct passage of the contents from one cell to the other. This 

 transverse wall, which may be either horizontal or oblique, is 

 known as the sieve plate, and the thin places, as pores of the 

 sieve. The sieve plates are sometimes also formed on the longi- 

 tudinal walls. When the activities of plants are suspended during 

 the winter, there is formed on either side of the sieve plates a layer 

 of a colorless, mucilaginous substance, known as callus, which has 

 somewhat the appearance of collenchyma, but is colored brownish 

 by chlor-zinc-iodide. 



The sieve cells contain an albuminous substance somewhat 

 resembling protoplasm ; in some instances starch grains have also 

 been found. 



When the activities of the sieve tubes have ceased, they be- 

 come altered in shape, and are then known as obliterated sieve. 

 In the drying of plants a similar alteration is produced, and for 

 this reason the sieve of vegetable drugs is of this character. 



