THE TISSUES OF PLANTS 



31 



generally perforated at the places where similar vessels 

 touch each other. The thickening, and as a consequence 

 the perforations, are of various kinds, but generally there 

 is a tendency in the former to the production of spiral 

 bands; this is more or less evident even when the bands 

 form a network. The transverse partitions, which may 

 be horizontal or oblique, are in some cases perforated with 

 small openings, in others they are almost or entirely ab- 

 sorbed. The diameter of the vessels is usually consider- 

 ably greater than that of the surrounding cells and ele- 

 ments of other tissues, and this alone in many eases may 

 serve to distinguish them. When young they contain 

 protoplasm, but as they become older this disappears, and 

 they then contain air. 



Tracheary tissue is found only in ferns and their rela- 

 tives and the flowering plants. The principal varieties of 

 vessels found in tracheai-y tissues are the following: 



51. (1) Spiral Vessels, which are usually long, with 

 fusiform extremities; their walls are thickened in a spiral 



Fig. 20.— Longitudinal section of a portion of the stem of Garden 

 Balsam (Impatiens). D, a ringed vessel; v\ a vessel with rings and short 

 spirals; v", a vessel with two spirals; v'" and v"'\ vessels with branch- 

 incr oTiirnio • n'"" n. irosaol with iiTKffiilar thickenings, forming the reticu- 



ing spirals . 

 lated vessel. 



, a vessel with irregular thickenings, forming the reticu- 

 (From Duchartre.) 



manner with one or more simple or branched bands or 



fibres (Fig. 30, v", 



V , V 



"). This form may be regarded 



