ELEMENTARY OEGANS. 



117 



.-; ivf) 



660, China regia. 

 Fibres cut transTersely. 



661. Clematis, 

 Punctata fibre. 



nearly filled up. The canal which forms its axis is cylindric ; but its outer walls, 

 which are pressed against those of the neighbouring fibres, are flattened and pris- 

 matic, as may be seen by a transverse section of fibrous tissue (fig. 660). 



The fibres, being spindle-shaped at their extremities, cannot be in 

 juxtaposition throughout their length, but the extremities of other fibres 

 are inserted between their free portions, and hermetically close the 

 conical interspaces above and below them (fig. 661). When the successive 

 inner layers completely line the outer layer, as frequently happens, the 

 cavity of the fibre remains smooth ; if the second layer does not com- 

 pletely line the first, spiral or reticulate thickenings are the result {spiral 

 or reticulate fibre) ; and dotted or punctate fibre (fig. 661), the most 

 common form of all, is the result of the failure of the 

 inner layer over minute areas. 



Vessels are much elongated tubes, the walls of 

 which are never smooth, but present either slender 

 spots or lines, or a close network, or rings, or spiral 

 lines ; they are cyhndric, and constricted at intervals 

 (fig. 668). The contractions are circular and hori- 

 zontal and close set, or oblique and distant. If 

 the vessel be boiled in dilute nitric acid, it breaks 

 up at the striEB. Where the constrictions occur, membranous folds often project 

 as 1 ings or perforated diaphragms into the interior ; whence it has been concluded 

 that the vessel is formed partly of cells,- partly of fibres 

 joined end to end, of which the ends, which at first 

 formed septa, have gradually become obliterated or per- 

 forated. The vessels, like the cells and fibres, are named, 

 according to the appearance of their walls, punctate, striate, 

 reticulate, annular, spiral. The spiral vessels, or trachece (fig. 

 662), are membranous tubes, uninterruptedly traversed 

 within by a pearly white spiral thread; this thread is 

 neither tubular nor channelled, but cylindric, flattened 

 (fig. 663), or a four-sided prism. The tracheae being 

 spindle-shaped at each end (fig. 662), are regarded as 

 elongated fibres. ISTothing is easier than to examine these 

 tracheae : if young shoots of Rose or Elder be gently broken, 

 there will be seen by the naked eye between the ruptured 

 662. Melon, surfaccs a spiral thread, lengthening and shortening like 

 ™° "^' a piece of elastic. The outer membrane is not so obvious, 

 except when the coils of the spiral thread are very remote. In most 

 cases the spiral thread is- single, but it may be double, and sometimes 

 as many as twenty form a ribbon {Banana) and can be unrolled together. Einally, a 

 spiral thread, which was originally single, may become folded and broken up into 

 finer threads {Beet-root). 



Annular vessels (fig. 664) are membranous tubes girt within by rings, which 



G65. Mamlllalia. 

 Trachea. 



