VASCULAR TISSUE. 



355 



more or less pointed at both extremities, and having their 

 walls strengthened by internal deposits. Occasionally, 

 however, the fibre is short, as in the Clematis, Elder, &c. ; 

 it is marked with pores or dots, from a deficiency of the 

 internal deposits at these points. 

 Vascular tissue consists of 

 (jells, more or less elongated, 

 joined end to end, or over- 

 lapping each other, in which 

 either a spiral fibre, or a mo- 

 dification of the same, has 

 been deposited ; hence, if the 

 spiral be perfect, it is called 

 a true spiral vessel ; if inter- 

 rupted, or the fibre breaks up 

 into rings, it is termed annu- 

 lar; if the rings are connected 

 together by branching fibres, 

 so that a network is pro- 

 duced, the vessel is called 

 reticulated; if the vertical 

 fibres are short, and equidis- 

 tant, the vessel is said to be 

 scalariform, from its resemblance to a ladder. Spiral 

 vessels have been also termed trachece, from their resem- 

 blance to the air-tubes of insects, as in fig. 198. 



Under this head other membranaceous tubes are included, 

 in which the arrangement of the fibre has been consider- 

 ably modified in its deposition. Elongated tubes or ducts, 

 with porous walls, come under the head of vascular tissue ; 

 they somewhat differ from the spiral varieties, inasmuch 

 as they cannot be unrolled without breaking. It is a 

 curious fact, that mostly the spiral coils from right to 

 left ; and it has been suggested that the direction of the 

 fibre may determine that in which the plant coils round 

 an upright pole. The Hop has left-handed spirals, and is 

 a left-handed climber, which would therefore appear to 

 support this theory. The nature of the fibre, and the 

 development of the tissue, have been frequently the subject 

 of dispu-te between botanists. 

 The late Mr. Edwin Quekett gave much attention to this 



AA2 



Fig. 188. Simple and compound 

 amral vessels. 



