32 



THE CELL AND PROTOPLASM 



Fig. 3. Piiritied and expanded cellulosie matrix 

 from section similar to Fig. 2. Total enlargement, 



X3600. 



Fig. 4. Lignin residue of section similar to Fig. 



2. Total enlargement, x 3000. 



shows conspicuous conceutricites or laniella- 

 tions, and, in addition, a finer radial pat- 

 tern that approaches the limits of micro- 

 scopic visibility. Fi<i'. 2, on the contrary, 

 exhibits a doiiiinantly radial and ramifying- 

 pattern. When such sections of the sec- 

 ondary Avail are carefully and ur;idually 



expanded by the use of suitable s^velling• 

 reagents, finer and finer details of the char- 

 acteristic structural patterns become suc- 

 cessively visible under the microscope (Figs. 

 3 to 6). In all cases the secondary wall 

 resolves itself ultimately into a system of 

 fine threads, or microfibrils, which are 

 variously aggregated and more or less ex- 

 tensively coalesced. Where the microfibrils 

 are oriented approximately parallel to the 

 long axis of a cell, they appear in expanded 

 transverse sections of such a cell as more 

 or less circular specks or granulations (Fig. 

 5). The concentric, radial, ramifying, and 

 other structural patterns of the secondary 

 wall are due, therefore, to varying densities 

 or porosities in different parts of the wall. 

 In the denser (darker) parts, the micro- 

 fibrils are more numerous per ^^nit area 

 and are more extensively coalesced, whereas 

 in the more porous (lighter) parts they are 

 less closely aggregated and the area of in- 

 terfibrillar capillary spaces is larger. 



In the case of a heavily lignified secon- 

 dary wall, it is possible to remove the cellu- 

 lose and leave a firmly coherent residue of 

 lignin; and, conversely, it is possible to re- 

 move the lignin and leave the coherent 

 matrix of cellulose. The cellulosie matrix 

 (Fig. 3) and the lignin residue (Fig. 4) 

 exhibit positive and negative images of 

 the same original, nnswollen structural pat- 

 tern (Fig. 2). It is evident that the lignin 

 residue may be interpolated within the 

 porosities of the cellulosie matrix. Such 

 facts as these indicate not only that lignin 

 occurs within the elongated porosities of the 

 cellulosie matrix, but also that the micro- 

 (•a]iillary spaces in cellulose are intereom- 

 miniicating. It is evident, accordingly, that 

 in dealing wdth the secondary wall of plant 

 cells we are concerned with two continuous 

 interpenetrating systems, (a) a matrix that 

 is composed of coalesced microfibrils^ and 

 (h) a system of elongated microcai-)illaries 

 that are interconnected. 



The microfibrils of the secondai'y wall are 

 c()nnnonly composed largely of cellulose, but 



' The term microfil)rils is used not necessarily to 

 designate actual discrete entities, but rather as a 

 convenient expression for describing the elongated 

 jiai'ts of a continuous, ]iorous matrix. 



