THE TISSUES OF PLANTS. 93 
Tracheary tissue is found only in ferns and their rela- 
tives and the flowering plants. The principal varieties of 
vessels found in tracheary tissues are the following: 
49. (1) Spiral Vessels, which are usually long, with fusi- 
form extremities; their walls are thickened in a spiral man- 
ner with one or more simple or branched bands or fibres 
(Fig. 15, v’’, v’’’, v’’”’), This form may be regarded as 
the typical form of the vessels of tracheary tissue. Ringed 
and reticulated vessels are opposite modifications of tl~ 
spiral form; the first are due to an under-development of 
Fic. 15.—Longitudinal section of a portion of the stem of Garden Balsam (Im- 
patiens). v, a ringed vessel; v’, a vessel with rings and short spirals; v’’, a ves- 
sel with two spirals; v’” and v’’”’, vessels with branchin spirals; v’””’, a vessel 
with irregular thickenings, forming the reticulated vessel. 
the thickening in the young vessels, resulting in the pro- 
duction here and there of isolated rings (Fig. 15, v); reticu- 
lated vessels are due, on the contrary, to an over-develop- 
ment, which gives rise to a complex branching and anas- 
tomosing of the spirals (Fig. 15, ’’””’). 
50. (2) Scalariform Vessels.—These are prismatic ves- 
sels whose walls are thickened in such a way as to form 
transverse ridges. They are wide in transverse diameter, 
and their extremities are fusiform or truncate (Fig. 16). 
51. (3) Pitted Vessels.—The walls of these vessels are 
thickened in such a way as to give rise to pits and dots, 
