MARKINGS OF THE WALLS OF CKLLS. 



39 



pith ; while the smaller dots, of the ordinary kind, as on the wood- 

 cells of the Plane-tree (Fig. 32), are most abundant on the sides that 

 look towards the centre and the circumference of the trunk. The 

 nature of these disc-like markings is plainly revealed in the accom- 

 panying microscopical dissections of Wliite-Pine wood (Fig. 36, 37). 

 They are thin places, which have not received the thickening deposit 

 that has lined all the rest of the calibre, or have received it in a 

 lesser degree. Those of contiguous wood-cells always exactly cor- 

 respond, just as do the smaller dots or pits of ordinary wood ; and 

 the two cell-membranes separate from each other, each being some- 

 what curved inward, thus leaving a lenticular space between them, 

 like that between two watch-glasses put together by their edges. 



47. Bands, Rings, or Spiral Markings. These are mostly 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 (GO), and particularly 

 by the remarkably thick plate which winds around in the cells of 

 certain Cacti, like a spiral staircase (Fig. 42, 43). The accompany- 

 ing figures illustrate various forms of banded, reticulated, or spiral 

 markings. 



48. When the primitive walls of such banded cells remain very 

 thin and delicate, they are apt to become obliterated at maturity, 

 leaving the firmer fibrous markings .as separate threads. This 



i'lG. 38. A cell of the pitlt of Elder, marked -with oblong dots, which are thin places. 



FIG. 39. Cells of the leaf of Sphagnum, or Peat-Moss, marked with a spiral fibre. 



FIG. 40 - 43. Spirally banded cells from species of Cactus, after Schleiden. 



FIG. 44. Hairs from the seed-coat of Dipteracanthus strepens ; one with a spiral band, the 

 other with a set of rings dcTeloped on the inner surface of the tube. 



FIG. 45. Tissue from the lining of the anther of Coha3a scandens ; where, the delicate walls 

 of the cells being soon oblitei'ated, nothing but the fibrous bands with which they were marked 

 remain. 



