370 G. ERDTMAN 



are direct descendants from pioneers (partially at least of an apocratic habit) 

 which, as described by Erdtman (1949) Uved on a terrain that protruded as a 

 small nunatak from the surrounding, rapidly shrinking continental ice. 

 Among the pioneers were some plants now confined, in their capacity of more 

 or less kinetophilous elements, to Oland, particularly on the Alvar steppe 

 {Helianthemum cf. oeJondicum, Artemisia cf. riipestris). Kinetophilous plants 

 like these, disUking much competition, probably had a chance of surviving, 

 e.g. in the sometimes almost vertical slopes of Mount Omberg facing Lake 

 Vattern. No traces of them have been found there, however: a continuous 

 existence as botanical "cat-burglars" over 10,000 years is not easily accom- 

 plished. 



Detailed studies of the pollen or spore morphology also in "trivial" 

 elements in the flora of Iceland and Scandinavia, etc., can lead to new, 

 unexpected vistas. Thus, at the Palynological Laboratory in Solna, about 

 10,000 pollen slides of Scandinavian plants have been made and distributed 

 to a number of scientific institutions in Sweden and Finland. Among these 

 were also slides of Rorippa silvestris. One Rorippa slide, however, was returned 

 with the comment that a mistake must have been made since pollen grains of 

 the same type as those in the slide "do not occur in the Cruciferae". A check 

 revealed that there was no mistake. Pollen grains from 40 different specimens 

 of Rorippa silvestris from various parts of Europe were investigated. The 

 grains in "statocratic" individuals were, it seemed, all normal, 3-colpate, 

 whereas those in more or less sterile individuals of a somewhat "apocratic" 

 habit (growing along roads, in grass swards in Stockholm and suburbs, etc.) 

 were deviating. In certain examples the pollen grains were devoid of apertures 

 or were provided with five to six colpi or colpoid apertures. In some specimens 

 the outer part of the exine (the sexine) was reticulate, in others retipilate, i.e. 

 provided with drumstick-shaped processes arranged in a reticuloid pattern. 

 In some grains, finally, the sexine was considerably thinner than the inner 

 part of the exine (the nexine). Other grains were, it seemed, destitute of 

 nexine or almost so. A cytotaxonomic analysis of the Rorippa silvestris- 

 complex is now being carried out at Uppsala (for further notes, illustrations, 

 etc., cf. Erdtman, 1958). 



I have deliberately dealt with Rorippa silvestris at some length, mainly for 

 two reasons. The first is that this palynological investigation revealed some- 

 thing actually going on before taxonomists and cytotaxonomists had ob- 

 served that there was anything "wrong" or peculiar with the Linnean species 

 Rorippa silvestris. The second reason is strictly palynological: which factors 

 are responsible for the variation of aperture patterns and the details of 

 exine stratification? This is a question of great interest. Critical investigations 

 into this theme would be much more worthwhile than uncritical speculations 

 on the taxonomic and phylogenetic bearing of certain pollen grain characters 

 now in vogue. 



