36 THE TISSUES OF PLANTS 



development, so that the conduction of the water is not 

 dependent upon the activity of these cells but occurs in 

 the cavities left empty by the disappearance of the proto- 

 plasm. Since the cells lack protoplasmic contents which 

 would furnish the turgor to keep them from collapsing, 

 the thickening of the walls is necessary. It often happens 

 that adjoining living cells swell out through the thinner 

 places into these cells, these bladder-like projections 

 being called tyloses. A distinction is made between 

 tracheids which are formed of single cells, and tracheae 

 (singular, trachea) or vessels, which are more or less 

 elongated tubes formed by the absorption of the cross 

 walls of adjoining cells so that the lumens of many suc- 

 cessive cells are all connected. The latter usually attain 

 the greater diameter. Tracheids are mostly not over 1 

 mm. long although in some cases they reach a length of 

 1 centimeter or even much more. Tracheae, accord- 

 ing to Strasburger, average about 10 centimeters long, 

 but in some cases reach a length of 2 to even 5 meters. 

 In some vines, the diameter reaches 0.3-0.7 mm. Trach- 

 eary tissue is found only in the higher plants, i.e., Seed 

 Plants and Ferns and Fern Allies. 

 47. In accordance with the character of the thickening, 

 there may be distinguished sev- 

 eral types of tracheary tissue, 

 these same types of thickening 

 being found both in tracheids 

 and tracheae. These are ringed 

 (or annular), spiral, reticulated 

 (netted), scalariform (ladder- 

 like) and pitted tracheae or tracheids. All but the last 

 are named after the manner of the internal thickenings of 

 the walls. The pitted cells, however, are those in which 

 the thickening is more extensive than in the others, the 



