ELEMENTAET STBUCTUBE. 15 



membranes, which give rise to the pitted cells already described), 

 consist of delicate threads or bands of varying thickness called 

 fibres, which assume a more or less spiral direction upon the inner 

 surface of the primary cell-membrane {figs. 29-31), and thus give 

 rise to what are c^WqH fibrous cells. In most cases the fibres are 

 wound to the right, although instances occasionally occur where 

 they turn in a contraiy direction. Such fibrous cells occur in 

 various plants and parts of plants ; thus in the leaf of the Sphag- 

 num, the hairs of many Cacti, in some of the membranes of 

 many seeds, as those of Salvia, Collomia, &c.; in the spore-cases 

 of some of the Flowerless Plants, in the inner lining of all an- 

 thers, in the outer rind of the aerial roots of many Orchids, 

 and in many other instances. 



These fibrous cells also present some differences of appearance 

 as regards the distribution of their fibres. Thus, in some cells 

 the fibre forms an uninterrupted spiral from one end to the 

 other {fig. 29) : such are termed spiral cells. In other cases the 

 fibre is interrupted at 



various points, and as- ^'9- 29. Fig. 30. Fig. 31. 

 sumes the form of rings 

 upon the inner surface 

 of the cell-wall {fig. 30), 

 and hence such cells are 

 called annular or ringed. 

 Instances also occur 

 even more frequent- 

 ly, in which the fibres 

 are so distributed as to 



produce a branched or f^g'. 29. Spiral cen. J'/sr. so. Annular or ringed 



^^ J / r' cell. Fig. 31. Ramified or reticulated cells. 



netted appearance \jig. 



31); such cells are termed ramified or reticulated. These annu- 

 lar and reticulated cells are merely modifications of the spiral, 

 as is sho^vn by the circumstance of our frequently finding in 

 the same cell intermediate conditions of all these forms. (For 

 further particulars on this head see Annular and Reticulated 

 Vessels.) 



The turns of the fibre, or the rings, may be nearly in contact, 

 or more or less separated by intervals of cell-membrane ; which 

 latter appearance is probably due to the growth of the membrane 

 after the deposition of the fibre. The turns of the fibre, or the 

 rings, again, may be either intimately attached to the cell-mem- 

 brane, or but slightly adherent, or altogether free. As a general 

 rule, the less the cell-membrane grows after the deposition of 

 the fibre, the more firmly is it attached to it. 



These different kinds of fibrous cells are connected by a 

 number of intermediate forms with the pitted cells already 

 treated of {fig. 32); and hence it has been supposed by many 

 that the secondary layers which give rise to the latter structures 



