POLLEN 161 



Orchidaceae. In the Mimoseae, pollen is often shed in masses of 8 to 64 

 grains. 



Structure of the Exine. The exine of the pollen grain is often sculptured 

 in many and elaborate patterns, which characterize major or minor taxa 

 (Fig. 63). Pollen grains that remain in tetrads have the surfaces of con- 

 tact with other spores of the tetrad smooth or only lightly sculptured 

 when exposed by enlargement at germination. The patterns have con- 

 siderable taxonomic and phylogenetic value. Projections in the form of 

 ridges, spines, and granules are the prominent features. The thin areas 

 between the projections are of two types: germinal furrows and ger- 

 minal apertures, or pores. The furrows, various in form and distinctness of 

 limit, represent elongate areas where the wall is thin and elastic. They 

 provide structural adaptation to changes in volume of the grain due 

 to changes in atmospheric humidity. The apertures are small thin areas, 



r\ 



Fig. 64. Diagrams to show the types of arrangement of pollen grains in the tetrad. 

 A, tetrahedral; B, cross; C, square; D, rhomboidal; E, linear. (After Erdtman, 1945.) 



which represent positions of possible emergence of pollen tubes. They 

 are usually located within the furrows but may be present in the thicker 

 wall areas. Apertures in some taxa are probably phylogenetically short- 

 ened furrows. In walls of elaborate structure, the apertures may be 

 covered with "caps" of thicker wall, which are readily lifted by pres- 

 sure of the developing pollen tube — Cucurbitaceae. Where no apertures 

 are present, the pollen tubes grow out from the furrows. Germinal fur- 

 rows are absent in some families, and rounded, thin areas, pores, take 

 their place functionally. These areas may be numerous — about thirty in 

 the Polygonaceae. Specialization of these basic types within the angio- 

 sperms has apparently followed several lines, with greatly modified types 

 resulting — both more elaborate and simpler in form. Similarity of type 

 has evolved by convergent changes in lines not closely related. 



Pollen-grain Types. There are apparently two major types of pollen 

 grains: monocolpate (Fig. 63B, G) and tricolpate (Fig. 63F, H), with 

 one and three germinal furrows, respectively. The monocolpate type is 

 elongate or rounded, with the furrow on the side not in contact with 

 the other grains in the tetrad. This seems to be the primitive form. It is 

 characteristic of the monocotyledons and most of the Ranales, espe- 

 cially the woody families — Winteraceae, Degeneriaceae, Magnoliaceae, 



