1368 A TEXTBOOK OF THEORETICAL BOTANY 



of the sculpturing. He points out that sculptured grains have a fairly heavy 

 coat of oil, which is absent from unsculptured grains. The extine is a 

 deposit from the polyphasic colloid sol of the degenerating tapetal proto- 

 plasm. The developing grains absorb the aqueous phase of the sol but the 

 oil is deposited as droplets on its surface. The droplets are separated from 

 each other at first by the remaining part of the aqueous phase and they are 

 free to adopt " least surface " configurations. Further deposition on the 

 extine takes place only in ridges between the droplets, until the grain dries 

 at maturity, when the droplets coalesce to form a continuous layer. The 

 type of sculpturing deposited will depend upon the number, size and 

 arrangement of the oil droplets. 



The oily coating makes the pollen somewhat adhesive, hence it tends to 

 cling to the anther, even after dehiscence, and will also cling easily to insects 

 which brush against it. Furthermore the oily covering is a useful protection 

 against wetting by rain. Wind-distributed pollens generally have no such 

 covering and are dry or " dusty ", thus favouring easy dispersal. 



Pollen forms have been made the subject of extensive study and an ela- 

 borate system of nomenclature has been devised by Potonie, Wodehouse, 

 Erdtman and others, to enable pollen morphology to be accurately described. 

 This is important from several points of view. Pollen may be used as evi- 

 dence for the presence of species, both in existing vegetation and more 

 especially in fossil deposits, where no other remains are available. The 

 quantitative pollen-analysis of the younger geological beds has thrown a 

 great deal of light on the history of the native vegetation in the countries 

 where it has been exploited and its extension to older strata may well assist 

 in building up a better picture of the early history of the Angiosperms. The 

 closely related subject of spore-analysis has also proved of great value to 

 geologists in the stratigraphy of Palaeozoic beds. Moreover the importance 

 of pollens as respiratory allergens has given them a medical interest, in 

 which accurate identification is equally necessary. Some of the results of 

 the pollen-analysis of Quaternary beds will be given at the end of the chapter 

 on Palaeobotany in Volume HE 



Radial symmetry prevails among pollen grains and this is easily under- 

 standable, since the grains are free in the loculus from an early stage and are 

 immersed in a fluid medium. Before the cutinization of the exo-extine they 

 are moulded by surface tension and are only rarely affected by the pressure 

 of surrounding cells. The pressure exerted in an isolated sphere by surface 

 tension varies inversely with its radius: P = 2 T/R, where T is the surface 

 tension in dynes per cm. ^ and R is the radius. In very small spheres this 

 pressure will obviously rise considerably and must have powerful effects. 

 Oval-shaped grains are commoner in the Monocotyledons than in the Di- 

 cotyledons, but there is no constant distinction of form between the classes. 

 In Monocotyledons the tetrad of grains is usually arranged in a single 

 plane, while in Dicotyledons they are usually in a tetrahedron. Mono- 

 cotyledonous grains are therefore typically boat-shaped, with bilateral 

 symmetry about two planes, longitudinal and transverse. Dicotyledonous 



