20 



FOEM AND AKEANGEMENT OF VESSELS. 



(reticulum, a net) ; and the name dotted is sometimes applied when 

 the fibre is so broken up as to leave small isolated portions adhering 

 to the membrane (fig. 62). In scalariform vessels (scala, a ladder), 

 there are short horizontal lines or bars, composed of fibre, arranged 

 along the sides of the tubes, at nearly equal distances, like the steps 

 of a ladder, and presenting a striated appearance. In some cases, as 

 in the Vine (fig. 63), they are composed of tubes united to each other 

 by thin, broad, oblique extremities ; at other times they taper like 

 spiral vessels. They generally assume a prismatic form, the angles 

 being unmarked by lines, as is seen in Ferns (fig. 64). 



Pitted Vbssels.^ — Another kind of vessel common in plants is the 

 pitted vessel, so called from the appearance of pits or depressions on its 

 surface. The tissue formed by pitted vessels has received the name 

 of Vasiform tissue, Pitted tissue, Bothrenchyma, or Taphrenchyma (j366§og 

 or rd,(pgog, a pit). The vessels are of large size, and are easily observed 

 in the Vine (fig. 65), Sugar Cane, Bamboo, Gourd 

 (fig. 116 ter), and other plants, in which the sap 

 circulates rapidly. They consist of cylinders more 

 or, Jess elongated, in which the thickening matter is 

 so deposited as to leave part of the membrane un- 

 covered, thus giving rise to the porous or pitted 

 appearance. The uncovered portions of membrane 

 are sometimes absorbed in old 

 vessels, and a direct communicar 

 tion is established between them. 

 The pits or so-called pores have 

 sometimes a bordered aspect, 

 which, according to Schleiden, 

 depends on air contained in -the 

 cavities between contiguous ves- 

 sels. Pitted or porous vessels 

 are usually united to each other 

 by a broad and often oblique 

 septum. 



This kind of vessel occasion- 

 ally presents a beaded appearance, as if formed by pitted cells, with 

 distinct constrictions at their point of union (fig. 67). This arti- 

 culated Bothrenchyma is by some considered as a form of cellular 

 tissue (fig. 22). To vessels exhibiting contractions of this kind, 

 whether spiral or pitted, the terms moniliform (monile, a necklace), or 

 vermiform (vermis, a worm), have been applied ; and the tissue com- 



Fig. 66. 



Fjg. 66. 



Fig. 67. 



Fig. 65. Pitted vessel (Bothrenchyma) from the Vine, showing its connection with woody 

 fibres, and the htoad septa or partitions of the vessel itself. Fig. 66. Pitted vessel from 

 Traveller's joy {Clematis Vitalba), Fig. 67. Moniliform pitted vessels from the Common 



Balsam. 



