CELLULAR TISSUE. 



41 



" common in aromatic trees " ; probably under forms scarcely if 

 at all distinct from ordinary dotted wood-cells. 



46. Bands, Rings, or Spiral Markings, These are, in most cases at 



least, definite portions of the wall more thickened than the rest ; 

 as is shown by the spiral vessel, where the secondary formation is 

 restricted to a delicate thread, capable of being unwound (60) ; 

 and particularly by the thick plate which winds around in the cells 

 of certain Cacti, like a spiral staircase (Fig. 29). Markings of this 

 kind (which are rarely thick and projecting as in the last exam- 

 ple) occur as rings (Fig. 43), or fragments of rings (Fig. 44), but 

 more frequently as spiral threads or bands (Fig. 26), sometimes as 

 branching threads (Fig. 27); all of which, however, exhibit a spiral 

 tendency. The elongated cells which form the hairs on the seeds 

 of many Acanthaceous plants exhibit these markings in great va- 

 riety. Two such cells from the same seed, one with a series of 

 rings, the other with a continuous spiral thread, are represented 

 in Fig. 31. Sometimes a band of fibres appears to ascend in 

 the same direction : occasionally two spiral threads seem to 

 wind in opposite directions ; and sometimes branching threads in- 

 osculate and form a kind of network on the membrane, as in Fig. 



28. Often the rings or turns of the spiral thread are nearly in 

 contact (Fig. 45) ; while as frequently they are separated more or 



FIG. 25. Cell of the pith of Elder, marked with oblong dotg. 



FIG. 26. Cells of the leaf of Sphagnum, or Peat Moss, marked with a spiral fibre. 



FIG. 27-30. Spirally banded cells from species of Cactus, after Schleiden. 



FIG. 31. Hairs from the seed-coat of Ruellia strepens ; one with a spiral band, the other 

 with a set of rings developed on the inner surface of the tube. 



FIG. 32. Tissue from the lining of the anther of Cobaea scandens ; where, the delicate walls 

 of the cells being soon obliterated, the fibrous bands with which they were marked remain. 



