24 



ORGANOGRAPHY. 



Fig. 45. Spiral cell. Fig. 46. Annular or ringed 



cell. Fig. 47. Ramifled or reticulated cells. 



the fibre forms an uninterrupted spiral from one end to the 

 other {fig. 45) : such are termed sinral cells. In otlier cases the 



fibre is interrupted at 

 Fig. 45. Fig. 46. Fig. 47. various points, and as- 



sumes the form of rings 

 upon the inner surface 

 of the cell-wall {fig. 46), 

 and hence such cells are 

 called a nnular or ringed. 

 Instances also occur even 

 more frequently, in which 

 the fibres are so distri- 

 buted as to produce a 

 branched or netted ap- 

 pearance {fig. 47); such 

 cells are termed r amified or reticulated. These annular and reti- 

 culated cells are merely moamcanons of the spiral, as is shown 

 by the circumstance of our frequently finding in the same cell 

 intermediate conditions of all these forms. (For further parti- 

 culars on this head see Annular and Eeticulated Vessels.) 



The fibres in most cases are wound from left to right, although 

 instances occur where they turn in a contrary direction. 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-membrane, 

 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. 48); 

 hence it has been supposed by many 

 that the secondary layers which 

 give rise to the latter structures 

 are also deposited originally in a 

 spiral direction, and in fact that all 

 secondary deposits have a tendency 

 to assume a spiral arrangement ; 

 others, however, explain such a formation by supposing a slight 

 enlargement of the primary cell-membrane to take place after 

 the secondary layer has been deposited, by which the latter is 

 stretched and ultimately broken into little holes or slits at 

 various points. 



In some cases, as in the Yew {fig. 49), in the Mezereon, and in 



Fig. 49. 



Fig. 48. Pitted and reticulated cell. 



F'ig. 49. Wood-cells of tlie Yew 



(Taxus baccata). After Mohl. 



