84 VEGETABLE HISTOLOGY. 



CHAPTER XX. 



TRACHEARY TISSUE. 



This includes tracheids and ducts. When speaking of the 

 fibro-vascular bundles, it was said that the xylem portion of 

 the bundle was that which contains the wood cells and tracheids 

 or ducts. The peculiarity of tracheary cells is that the walls 

 are thickened unevenly, the thick parts being on the inside of 

 the walls and arranged in various forms, giving rise to spiral, 

 reticulate, scalariform, annular, dotted ducts, etc. 



The cells of tracheary tissue are usually less thick-walled 

 than wood fibres and of larger diameter, and mostly oblique- 

 ended or blunt. They are more widely distributed than wood 

 fibres, as they are found in all vascular plants, i. e., plants con- 

 taining bundles. (The name fibro-vascular means that there 

 are both fibres and vessels or ducts in the bundles.) 



Ducts and tracheids are alike in regard to the markings of 

 the walls. The difference is that a duct is composed of a row 

 of tracheids in which the separating partitions have been ab- 

 sorbed, leaving a duct or vessel. A tracheid is a single cell 

 complete on all sides. The name tracheary signifies the resem- 

 blance between some of the ducts and the trachea or wind-pipe 

 of man. 



TRACHEARY TISSUE IN GERANIUM STEM. 



Make longitudinal radial sections and macerate in Schulze's 

 solution, wash and stain in methyl-green, mount in water. The 

 following ducts may be found in the xylem of the bundles, 

 especially if the cells be separated by tapping upon the cover 

 glass. 



KETICULATE DUCTS. So called because the thickenings in the 

 cell-wall are in the form of a network, giving a pitted appear- 

 ance to the wall. The walls are somewhat prismatic and the 

 pits occur on the flat sides. By means of them a lateral osmotic 

 circulation is kept up with other ducts. Notice in some of the 

 ducts, on the oblique ends, large openings where one cell com- 

 municates with another. Reticulate ducts are the most 

 numerous in the Geranium, as well as most plants (Fig. 51). 



SPIRAL DUCTS. Widely distributed in plants. The thicken- 

 ing in the wall takes place in a perfect spiral, the part between 

 the turns being very thin and almost invisible and cellulose in 

 nature. There may be two spirals in a duct, one within the 

 other. In other plants ducts with more than two spirals are 

 found. Sometimes the turns of the spirals are connected by 

 cross-thickenings, and then the duct merges into a reticulate 

 one. Again, the thickenings may be spiral on one end of the 



