VASCULAR TISSUE. 



47 



places than the rest of the wall. They are elongated cells, or tubes 

 formed by the confluence of several cells into one, with the delicate 

 walls strengthened by the 

 deposition on their inner 

 surface of additional ma- 

 terial, in the foi-m of bands, 

 sometimes branching and 

 forming network (the Re- 

 ticulated duct), as in the 

 middle of Fig. 60, or of 

 rings (the Annular duct), 

 \ as in the middle of Fig. 

 61, or of a continuous spi- 

 ral thread (Fig. 62, 63), or 

 a number of such threads 

 ^ (Fig. 64), thus forming the 

 .Spiral duct or Spiral vessel. The coiled thread has been generally 

 thought to be solid. But Trecul, in a memoir already referred to 

 (42, note), insists that it is hollow, and it really appears to be so in 

 the thick threads or bands of certain cells in the wood of several 

 sorts of Cactus, such as are shown in Fig. 40 - 43, 

 which are weU adapted for the investigation of 

 this point. In the true Spiral Vessel the fibre is 

 so strong and tough, in comparison with the deli- 

 cate membrane on which it is deposited, that it 

 may be torn out and uncoUed when the vessel is 

 pulled asunder, the cell-wall being destroyed in 

 the operation. This is seen by breaking almost 

 any young shoot or leafstalk, or the leaf of an 

 Amaryllis, and gently separating the broken ends ; 

 when the uncoiled threads appear to the naked 



V FIG. 60. A portion of a duet from the leafstalk of Celery ; the lower part annvlar ; the 

 middle reticulated, and the thread at the upper part broken up into short pieces. 



FIG. 61. Duct from the Wild Balsam or Jewel-weed } the coils of the thread distant j a 

 portion forming separate rings. 



FIG. 62. A simple spiral vessel, torn across, with the thread uncoiling. 



FIG. 63. Two such vessels joined at their pointed extremities. 



FIG. 64. A compound spiral vessel, partly uncoiled, from the Banana. 



FIG. 65. A bundle of spiral ducts from the stem of Prince's Feather (Polygonum orientale), 

 magnified : a, one composed of short cells and with the fibre closely coiled : the next, 6, is 

 composed of much longer joints, and has a very loose coil : c is short-jointed, and the fibre of 

 the loose coil is occasionally forked : d and e show no appearance of joints or partitions, and 

 the turns of the spiral fibre are still more remote. 



