INVESTIGATION OF STRUCTURE IN PLANT CELL WALLS 65 



cross, whose arms are parallel to the directions of vibration in the 

 polarizing prisms. The upper right and lower left sectors are coloured 

 green, i.e. show an addition colour, while the upper left and lower right 

 are in subtraction colour. Along the arms of the cross, therefore, the 

 m.e.p. of the starch crystallites must he either parallel or perpendicular 

 to the arms of the cross, i.e. radially or tangentially in the grain. Since 

 the upper right-hand sector is green, and the grain must be symmetrical, 

 it follows that the m.e.p. must lie radially. Here, then, all the colour 

 effects shown by a cell wall on rotation are manifested without rotation. 

 It should be clear that if the grain is rotated, the cross and the sectors 

 remain stationary. Interpretation of the larger grains is now easy. The 

 same phenomena are shown, but they are slightly distorted on account 

 of the eccentricity of the grain: in every case the m.e.p. of the crystallites 

 (and therefore presumably the long molecular chains of amylose) lie 

 radially to the lamellae. Plate III, Fig. 3 shows a somewhat similar 

 type of structure in bordered pits of conifer tracheids. Comparison 

 with Plate III, Fig. 2 will make it clear, however, that here the particles 

 (this time of cellulose) lie tangentially to the edge of the border. 



With the particular wall object examined so far, it would be found 

 that the m.e.p. is exactly parallel to the edge of the wall. This is quite 

 a general rule for walls seen edgeways. Since the m.e.p. represents the 

 projection of the cellulose chain direction in the plane of the section, it 

 follows that the chains always lie flat in the surface of the wall. This is 

 presumably a consequence of their deposition at the surface of the 

 cytoplasm. 



Returning to Fig. 28, suppose now attention is paid to the area A, 

 where the top wall of the cell has been removed so that a single wall 

 is observed in face view (this can be done quite readily, see p. 116). 

 Then it wiU generally be found in elongated cells that the m.e.p. is 

 tilted to the cell length along, say, LM. The angle d can then be deter- 

 mined. A moment's thought will add the further information that, 

 considering the ceU as a whole, the m.e.p. lies along a spiral defined by 

 the angle d. This must be true since: 



{a) However the cell is cut, the m.e.p. is always tihed in the same 

 direction or, what is actually observed, if a whole population of cells 

 are cut in this way, then all the m.e.p.s are usually tihed in the same 

 direction. 



{b) If the double wall at D be examined, then it will be found that 



its m.e.p. can be determined only approximately, but it lies very nearly 



parallel (or perpendicular depending on whether d is less or greater than 



45°) to the length of the cell. It will be clear from Fig. 31 that this means 



5 



