132 



e.g. water). When the OPD is known, the dry mass (m), as shown by 

 Davies, Wilkins, Chayen and La (',our (1954), can be calculated from the 

 following formula: 



X 

 (I)^ = OPD in water; A =area cm^; ^ for a dry object can be calculated from 

 the formula x = ~^ where ^^ is the density of the object, /]„ is the refrac- 



tive index of the object, and n^ that of water. 



In cell substances x usually varies between 0.14 and 0.19. A x value of 

 0.18 will give dry weight determinations with an error not exceeding 10 %. 

 If the cell consists of a substance of one kind only, the error will be greater; 

 in a mixture it will be less. In cells with plenty of storage products, e.g. 

 many kinds of pollen grains and spores, it may not be advisable to use the 

 average value of x- 



The thickness of the preparation or cell (/), is calculated from the formula 



All interferometric methods provide about the same accuracy. The OPD 

 measurements of homogeneous, phase-even objects can be made with a 

 precision of +20 A (+2 x 10^^ Q^n). In measurements of heterogeneous 

 objects the same precision cannot be attained. Due to diffraction effects 

 measurements of areas smaller than about 5 fi^ are not advisable. It is 

 thus possible to make determinations of dry weight of 0.05 pg (0.01 pg//<^). 



In order to get the best resolution the Dyson interference microscope 

 and that by Baker have a condenser with a large numerical aperture. The 

 light traverses the object obliquely and the measurement values therefore 

 tend to be slightly too high (Ingelstam and Johansson 1957). In another 

 interference microscope described by Johansson and Afzelius (1956) and 

 Johansson (1957), parallel light, perpendicularly passing the object, is 

 used. Thereby a greater precision in OPD measurements is achieved. 



Measurements of the mass difference at various stages of pollen grain 

 development, as well as at various stages of enzymatic digestion of pollen 

 grains have been made by Davies et al. (1954). Measurements of the OPD 

 of the exine, inline, cytoplasm, and nucleus of pollen grains have also been 

 made (Johansson and Afzelius 1956). 



Besides the interferometric methods, quantitative cytochemical data 

 may be obtained in other ways. Engstrom (1951) and Engstrom and Lind- 

 strom (1947, 1950) have measured the mass of cells by means of soft X-rays. 

 The method has also been tried on pollen material by Dahl and Engstrom 

 (1954). Davies, Engstrom, and Lindstrom (1953) comment upon the X-ray 



