192 



INTRODUCTION TO CYTOLOGY 



the pit is readily altered. Probably because of changes in pressure it 

 swings to the side of the pit; the torus then lies against the pit opening, 

 or "mouth," and the pit is blocked except for slow diffusion through the 

 rather thick torus. The latter may even be forced tightly into the pit 

 mouth. 



The secondary wall layer may be even more limited in extent, only a 

 small portion of the primary wall being covered. Such is the case in 

 protoxylem cells, in which the secondary layer is deposited in the form of 

 rings and spirals (Fig. 4). This form of thickening, together with the 



FIG. 67. FIG. 68. 



FIG. 67. Pits in the wood of Larix, showing perforations in pit membrane. X 



800. 



(After Bailey.) 



FIG. 68. Diagram of bordered pit of coniferous wood. 



A, section of pit showing closing membrane supporting the torus, and secondary layers 

 on each side of middle lamella. B, face view of same. C, section showing torus forced 

 against mouth of pit. (After Bailey.) 



peculiarly extensible character of their primary walls, allows for the great 

 increase in length of these cells necessitated by the continued growth of 

 the young organs in which they chiefly function. In some cells, notably 

 the tracheids of certain gymnosperms and the vessels of many angio- 

 sperms, a tertiary layer is deposited upon the secondary wall. This ter- 

 tiary layer takes the form of slender spirals, rings, and other figures 

 resembling the secondary thickenings of protoxylem cells. 



The Physical Nature of the Cell Wall. Hugo von Mohl (1853, 1858) 

 first expressed the idea that the cell wall grows by apposition, i.e., by,, the 

 deposition of material in successive laminae. Although certain other 

 workers (Wigand 1856) supported this view, it became over-shadowed for 

 a time by the theory of Nageli. This investigator, as a result of his classic 

 researches on the wall and on starch grains (1858, 1862, 1863), concluded 



