THE WALLS OF I'LANT CELLS 



39 



transverse sections, and ai-c soIuIjIc in 

 reapents which (h) not dissolve cellnlose. On 

 the contrary, tlie broad, ai)])arentiy isoti'opic 

 layers of Fi^s. ]4 and IS ai-c striidiitily hire- 

 frinj^'ent in hjiipitudinal sections, and the 

 tennons layers that ai-e birefrin<i('n1 in 

 transverse sections become isotropic or 

 feebly anisotropic. The intensity of bire- 

 fringence fluctuates in diagonal sections 

 with changes in the angle of obli(inity. This 

 is true regai-dless of wliether the sections are 

 examined under the polarizing microscope 

 before or after treating the sections, with 

 solvents for non-cellulosic constituents. It 

 may be determined by direct observation, 

 and by the various lines of corroborative 

 evidence outlined in the preceding para- 

 graph, that the microfibrils and micro- 

 capillaries of the tenuous layers are oriented 

 transversely or in helices of relatively low 

 slope, whereas those of the broad layers are 

 arranged longitudinally or in helices of very 

 steep slopes. In native cellulose, the major 

 axes of swelling anisotropy are oriented at 

 right angles to the long axis of the micelles 

 and microfibrils. In other words, a layer 

 of the secondary wall expands considerably 

 in planes at right angles to the long axis of 

 the microfibrils, but only slightly in planes 

 parallel to this axis. Thus, in walls having 

 the type of structure illustrated in Figs. 14 

 and 18, the broad central layers expand 

 laterally. The tenuous layers, which en- 

 velop these layers externally, have a nearly 

 transverse orientation of microfibrils and 

 are unable to increase markedly in circum- 

 ference. Therefore, they tend to rupture 

 during swelling of the secondary wall, as 

 shown in Fig. 24. 



Although the cellulosic matrix of the 

 secondary wall appears to be a continuous 

 rather than a discontinuous system, it ex- 

 hibits predetermined planes of hydrolysis 

 and of cleavage. The enzymatic hydrolysis 

 of cellulosic walls (Fig. 21) which is caused 

 by the activity of certain remarkable wood- 

 destroying fungi, progresses along tAVO 

 predetermined planes, as does the acetyla- 

 tion of cellulose and its hydrolysis by min- 

 eral acids. One of these sets of planes is 

 oriented parallel to the long axis of the 



iiii('i-()(il)i'ils n\](\, jDiri jxissii, of llie micro- 

 (•a])illaries, micelles, and chain molecules. 

 The other is oriented at an angle of 20 to 

 25'' to this axis. Tlie latter set of planes 

 of liydi'olysis and of chemical reaction is 

 not coiTclated with any \isible structure 

 of the cellulosic nuitrix, hut it does corre- 

 spond i-athei- closely with certain spacings 

 within the crystal-lattice of cellulose. These 

 spacings are between the easily hydi-olyzable 

 ether-liid^ages of the cube-center ami cube- 

 side chains. Tliey fluctuate arouml 14 A, 

 and form i)lanes which intersect the long 

 axis of the chain molecules at angles of 

 from 22° 40' to 25° 04'. Although the sub- 

 microscopic and the visible planes are 

 oriented at similar angles, it is not evident 

 why the chemical reactions should progress 

 along these specific planes. If it be assumed 

 that the spacing between the ether-linkages 

 is of most favorable magnitude for the in- 

 sertion and activities of enzyme molecules, it 

 is not clear why the reactions due to acetyla- 

 tioiL or to hydrolysis by phosplioric or sul- 

 phui-ic acids — Avhere smaller molecules are 

 concerned — should progress along the same 

 planes. Nor is it evident why the enzymatic 

 hydrolysis sliould progress so commonly 

 along a single plane rather than in a zigzag 

 manner, since at any specific ether-linkage 

 it Avould seem that hydrolysis might pro- 

 gress in various directions, either upward or 

 downward at angles of from 22 to 25°. 

 Therefore, the most that may be concluded 

 at present is that there are predetermined 

 planes of chemical reaction in native cellu- 

 lose, certain of which are closely correlated 

 with visible orientations in the cellulosic 

 matrix and others which must be caused 

 solely by molecular configurations. Sim- 

 ilarly, in mechanically-induced cracking of 

 the cell wall, there are predetermined planes 

 of cleavage, some of which may be correlated 

 with visible weaknesses in the cellulosic 

 matrix, and others which are caused by 

 submicroscopic factors. One of these sets 

 of planes of structural weakness is oriented 

 parallel to the long axis of the microcapil- 

 laries and microfibrils and is much accen- 

 tuated in cell walls having concentric (Fig. 

 5) or radial lamellae (Fig. 2) of strikingly 



