VASCULAK TISSUE. 



355 



raore 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 

 cells, 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, fiom its resemblance to a ladder. Spiral 

 vessels have been also termed tracheae, 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 unroiled 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-lmnded 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 dispute between botanists. 

 The late Mr. Edwin Quekett gave much attention to this 



Fig. 198. —S/riral vessels from 

 Rhubarb. 



A A 2 



