FOBM AND ARRANGEMENT OF VESSELS. 



17 



small space in the centre, as in some of the Cactus tribe. When 

 separate fibres cross each other, forming a kind of net- work on the walls 

 of the tubes (fig. 59), the vessels become reticulated (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. 60). 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. 

 61), they are composed of tubes united to each other by thin, broad, 

 oblique extremities; at other tunes they taper like spiral vessels. 

 They generally assume a prismatic form, the angles being unmarked 

 by lines, as is seen in Ferns (fig. 62). 



67 



58 



62 



36. Porous Vessels. Another kind of vessel common in plants is the 

 porous vessel, so called from the appearance of pores on its surface. 

 The tissue formed by porous vessels has received the name of Vasiform 

 tissue, Pitted tissue, Bothrenchyma, or Taphrenchyma (Soffpeg or Ta<pjso?, 

 a pit). The vessels are of large size, and are easily observed in the 

 Vine (fig. 63), Sugar Cane, Bamboo, Gourd (fig. 98 ter), and other 



Figs. 56, 57, 58. Annular vessels from the stem of the Common Balsam. 

 Fig. 59. Spiral vessel. Wide coil, and fibre dividing. 

 Fig. 60. Vessel showing rings of fibre and dots. 

 Fig. 61. Scalariform vessel from the Vine. 



Fig. 62. Prismatic scalariform vessel from Eoyal Fern (Osmunda reyalis). 

 C 



