64 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



grains from potato is given in Plate III, Fig. 2. The grains are all 

 coloured, and the variation in colour is at first sight bewildering. Care- 

 ful observation of the details readily gives, however, some useful 

 information as to their structure. Suppose attention is first confined 

 to the smaller spherical grains. Then each grain shows a red maltese 



Fig. 30. The use of a colour plate in determinations of the m.e.p. 



(a) The colour plate (Red I) is inserted between crossed Nicols in the 45° position. 

 The field is red. 



ib) The wall is superposed on the plate and its ny direction is parallel to that of the 

 plate, when the slow vibration in the plate remains the slow vibration in 

 the wall. Hence the path difference of (wall plus plate) is greater than that of the 

 plate alone. The path differences are therefore additive, and an addition colour, 

 blue or green, is shown. 



(c) If the wall is now turned through 90° the slow vibration in the plate becomes the 

 fast vibration in the wall, and therefore overtakes the other vibration slightly. 

 The total path difference is therefore less than that of the plate alone, and the 

 colour is a subtraction colour (orange or yellow). 



(d) In an intermediate position where the tty (or ««) position is parallel to PP, the 

 wall has no effect on the light and the wall is the same colour as the field. 



