THE TISSUES OF PLANTS. 



23 



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 



Fio. 15. — Loneritudinal section of a portion of the stem of Garden Balsam (Im- 

 patiens). v, a ringed vessel; v\ a vessel with ringrs and short spirals; u", a ves- 

 sel with two spirals; v"' and d"", vessels with branching 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. \5,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, v"'"). 



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. 



