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- 
=|2= SVE TT eral types. of tracheary tissue, 
2 —=)5\18 + these same types of thickening 
= ab iL rail being found both in tracheids 
7 eae: wills ST Bag (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 
and tracheae. ‘These are ringed — 
