which are difficult to draw and hard to describe. Microtome sections (cf 

 particularly Afzelius in Grana palynologica, 1:2, 1956) make these subtle 

 details of pollen construction easier to observe and safer to interpret. 



Near the proximal root of the sacci are often found slight, sexinous ridges 

 or frill-like projections (proximal crests, cristae proximales, also referred 

 to as cristae marginales) varying in appearance in different species. At the 

 distal root of the sacci, where these merge into the distal aposaccium, the 

 characteristic pattern of the bladders comes abruptly to an end. 



The height of the corpus coincides with the polar axis (i.e. the perpen- 

 dicular line connecting the poles); the breadth is identical with its maximum 

 horizontal extension in grains in equatorial longitudinal view (marginal 

 crests extending beyond the general surface of the corpus not included), 

 and the depth (in bilateral grains) is equal to the transverse ("non-sacci- 

 ferous") diameter of the corpus. It is often preferable and, at the same 

 time, easier to calculate the inner dimensions of the corpus. 



The height of a saccus is the shortest distance from the highest point of 

 the saccus (or from a line drawn through this point parallel to the saccale 

 nexine) to the underlying nexine of the corpus. Its breadth is equivalent 

 to the "tangential" diameter of the saccus in pollen grains in polar view. 

 In radiosymmetric grains the breadth can also be measured in pollen grains 

 seen in equatorial view; bilateral grains must be in a transverse equatorial 

 position if the breadth shall be measured. In microscope slides this, how- 

 ever, is seldom the case. Its depth— in bilateral grains— is equal to the 

 maximum diameter of the saccus in grains in equatorial longitudinal view. 

 In radiosymmetric grains the depth is calculated is a similar way. Height, 

 breadth, and depth of corpus and sacci are illustrated in Figs. 2, 3, 13, 23, 

 27, 43, 53, 57, 58, and 62. 



As shown by Afzelius the inner part of the nexine (the endonexine) in Ce- 

 drus is laminated (Fig. 12, p. 11). In acetolyzed pollen grains of this genus, 

 and of Abies, etc., it can often be seen, even by means of an ordinary 

 microscope, that the nexine consists of two distinct layers, which often 

 split apart as a result of the chemical treatment. So striking is this feature 

 that it seems extraordinary that it has not been mentioned until now. 



The morphology of bisaccate pollen grains has been dealt with by nu- 

 merous botanists, among whom was Strasburger, who believed that the 

 floor of the sacci was formed by intine. This opinion has often been echoed 

 right up to recent years, although Strasburger himself soon corrected his 

 mistake. 



In conclusion, it ought to be mentioned that, according to Ciguriaeva, 

 the thin ends (with sexine and nexine slightly separate from each other) 

 of the pollen grains in Ephedra and Welwitschia may be interpreted as the 

 remainder of true sacci. 



