CYTOKINESIS AND THE CELL WALL 175 



shown to be a fluid film, and it now seems evident that the deposition of 

 salts and other matter in this film transforms it into the middle lamella. 

 Whatever its origin, the middle lamella may increase somewhat in thick- 

 ness before distinct secondary layers appear. The finer structure of the 

 middle lamella is not adequately known, but there is much to suggest 

 that it consists in reality of two or three differentiated layers, through 

 one of which the lamella splits when intercellular spaces are developed 

 by a rounding up of the cells. 



It is probable that the deposition of the secondary layer begins after 

 the cell has reached nearly or quite its full size, though to this there are 

 apparently certain exceptions. The secondary layer, which seems to be 

 formed with considerable rapidity, differs from the primary layer chemi- 

 cally and in structure. There are circular or elongated areas in which 

 no secondary substance is deposited, so that the cells at these places are 

 separated only by the delicate primary membrane. Such a wall is said 

 to be "pitted," the primary layer extending across the pit being termed 

 the closing membrane. The central portion of this membrane (vascular 

 cells of gymnosperms chiefly) has sometimes a more or less conspicuous 

 thickening known as the torus. The portion of the membrane around 

 the torus is pierced by fine pores. In some cases these may become so 

 large and numerous that the torus appears to be suspended on a mesh- 

 work, while extreme cases are known in which it is held in place only 

 by a few strands. In bordered pits the secondary wall overarches the 

 margins of the closing membrane. In this type of pit, characteristic 

 chiefly of water-conducting cells of the gymnosperms, the closing mem- 

 brane is of such a nature that its position in the center of 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 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, where the secondary layer is deposited in the form of 



rings and spirals (Fig. 8, B, C). This form of thickening, together with 



the peculiarly extensible character of their primary walls, permits 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 angiosperms, a tertiary layer is deposited upon the secondary one. 



This tertiary layer takes the form of slender spirals, rings, and other 



figures resembling the secondary thickenings of protoxylem cells. ^^ It 



has been shown in the protoxylem and metaxylem cells of the gourd 



1* For a more complete account of such differentiations, see Eames and Mac- 

 Daniels (1925). 



