FOEM AND ARRANGEMENT OF VESSELS. 



19 



that of the stem. The coils of the spiral fibre may be close together 

 (fig. 52), or be separated (fig. 57). Sometimes ■ they become united 

 together, and to the membrane of the tube, so that they cannot be 

 imroUed. Such vessels are called closed trachese, or closed ducts, and 

 are -seen in ferns. 



False or Spueious Teacher, the ducts of some authors, are 

 vessels in which the internal fibre does not form a complete spiral 

 coil. The chief varieties are annular, reticulated, and scalariform 

 or ducts. In annular vessels (annulus, a ring), the fibres 



Fig. 58. 



Fig. 69. 



Fig. 60. 



Fig. 64. 



Fig. 63. 



form complete rings round the tubes (fig. 58). They resemble the 

 tracheae of animals more than spiral vessels do. The rings are by no 

 means regular ; they may be horizontal or inclined, simple or forked 

 (fig. 59), placed near to each other or separated by considerable 

 intervals, the intermediate spaces being sometimes occupied by a 

 fibre of an elongated spiral form, which is continuous with the rings 

 or distinct from them (fig. 60). All these forms are easily recognised 

 in the common Balsam. Occasionally, the ring becomes very much 

 thickened in a direction perpendicular to the walls of the vessel, so as 

 to leave only a small space in the centre, as in some of the Cactus 

 tribe. When separate fibres cross each other, forming a kind of net- 

 work on the walls of the tubes (fig. 61), the vessels become reticulated 



Figs. 58, 59, 60. Annular vessels from the stem of the Common Balsam. Fig. 61. 



Spiral vessel. Wide coil, and fibre dividing. Fig. 62. Ves.sel showing rings of fibre and 

 dots. Pig. 63. Scalariform vessel from the Vine. Fig. 64. Prismatic scalariform 



vessel from Boyal Fern (Osmwida regcdis). 



