Chapter 5 



POLLEN 



In most angiosperms, germination of the microspore begins when 

 the flower bud is small, with the enlargement of the spore and the first 

 cell division within the spore. After one or two divisions within the spore 

 wall, a resting period usually occurs. The spore wall, together with its 

 contents of two or three cells in a dormant state, (an early stage of the 

 male gametophy te ) , is the pollen grain (Fig. 62F). The microspore is 

 mature long before the dehiscence of the anther, often while the anther 

 is still essentially sessile and the filament undeveloped. In plants that 

 flower in early spring, the anthers usually pass the winter with the 

 sporogenous tissue in the mother-cell stage or, less commonly, in the 

 microspore stage; rarely, the microspore has germinated. In these 

 plants, development may continue in the warmer periods during the 

 winter. Collectively, masses of pollen grains — individual grains or clus- 

 ters of various sizes — constitute the pollen. Where the grains formed 

 from the spores of a tetrad remain permanently together, the grains 

 have been called "compound pollen grains" but are better described as 

 "pollen grains in tetrads." 



Pollen grains show great variety of form, size, and sculpturing of the 

 wall (Fig. 63). In form, they are usually globose, ellipsoid, or fusiform, 

 but they may be lobed or angular. Their shape depends, in part, on their 

 moisture content. Drying may greatly change their form, but, when 

 moist, the grains will return to their original shape. Pollen may be shed 

 in either the moist or dry state: that of many Rosaceae is described as 

 shed in the moist state; that of the Compositae, in the dry state. The 

 most extreme shape is confervoid — as in many submersed aquatic genera, 

 Zostera, Thalassia, Fosidonia, Rtippia (crescent-shaped). Unusually 

 large grains characterize the Cucurbitaceae and very small ones some of 

 the Boraginaceae. 



Arrangement of Pollen Grains in the Tetrad. The pollen grains formed 

 by a mother cell are associated in several ways (Fig. 64). The most 

 common arrangements are the tetrahedral — the grains lying at the four 

 corners of a tetrahedron — and the tetragonal — the grains lying at the 

 corners of a square or rhombus. The tetrahedral arrangement is com- 

 mon in the higher dicotyledons. In the formation of the spores, the 

 tetrahedral arrangement usually results from simultaneous divisions; 



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