THE ACHROMATIC FIGURE, CYTOKINESIS, AND CELL WALL 191 



sporocytes of Larix and the root cells of Allium the connecting fibers 

 first thicken near the nuclei, then become uniform throughout their 

 length, and finally become swollen at the equatorial region, indicating a 

 transfer toward that region of the material that is to compose the cell 

 plate. Allen was able to show not only that the middle lamella itself 

 may increase in thickness by the addition of new material before the 

 secondary layers begin to be laid down, but also 

 that it consists in reality of two layers representing 

 the secretions contributed by the two daughter 

 protoplasts. Where these two masses of secreted 

 material meet there is developed a median plane of 

 weakness which is ordinarily invisible but along 

 which the lamella invariably splits when inter- 

 cellular spaces are developed by the rounding up 

 of the cells. By the use of proper staining methods 

 it has been found possible to differentiate this 

 " primary cleavage plane." The continuity of the 

 middle lamella is interrupted, if at all, only by 

 the fine pores through which pass the protoplasmic 

 strands connecting adjacent cells. (See p. 46.) 



Secondary and Tertiary Wall Layers (Fig. 66). 

 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 ap- 

 parently certain exceptions. The secondary layer, 

 which seems to be formed with considerable 

 rapidity, differs from the primary layer not only 

 chemically (see below) but also in structure, being 

 interrupted by 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 lamella extending 

 across the pit being termed the closing membrane. 

 The central portion of this membrane sometimes 

 (vascular cells of gymnosperms chiefly) has 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 meshwork (Fig. 67), while extreme cases are known in which it is 

 held in place only by a few strands. In bordered pits (Fig. 68) the second- 

 ary wall overarches the margins of the closing membrane. In this type 

 of pit, characteristic chiefly of water-conducting cells of the gymnosperms, 

 the closing membrane is of such a nature that its position in the center of 



j 



r 



FIG. 66. Longitudinal 

 and transverse sections 

 of a gymnosperm tra- 

 cheid; p, primary wall 

 or middle lamella; s, 

 secondary layer; t, spiral 

 tertiary thickening. 



