POLLEN 



163 



orately sculptured pollen, but, within the family, they show a series in 

 simplification toward loss of sculpturing in the anemophilous genera 

 (Fig. 65). Artemisia has smooth pollen; Xanthiiim has lightly sculptiu-ed 

 walls, with weak spines. This series is considered evidence that ane- 

 mophily has been recently established in the Compositae. Scattered, 

 "vestigial" patches of an adhesive layer on wind-borae pollen have 

 been considered evidence of the derivation of anemophily from en- 

 tomophily. The sculpturing of the exine is clearly correlated with method 

 of pollination. 



Fig. 65. Pollen grains of Ambrosieae. Diagrammatic equatorial sections showing pro- 

 gressive thinning of exine and reduction in size of spines in a phylctic series from 

 insect to wind pollination. A, Oxytcnia acerosa; B, Chorisiva nevadcnsis; C, Cijcla- 

 chaena xanthifolia; D, Ambrosia elatior; E, Xanthiiim speciusum. (After Wodchoiise, 

 1935. ) 



Pollen-grain Development. Three types of pollen-grain development 

 have been distinguished: the normal, Triglochin, and Jimcus types. In 

 the normal type, characteristic of most angiosperms, the spore enlarges 

 greatly in volume before dividing to form the generative and tube cells 

 (Figs. 62C, D and 66A, B). In the Triglochin type (Fig. 66A to D), cell 

 division occurs in the spore before there has been much increase in 

 size; the major increase in size follows the first cell division. The 

 Juncus type, restricted to the Juncaceae and Cyperaceae, seems in- 

 sufficiently distinct from the other two. The Triglochin type is present in 

 lower monocotyledons — Najas, Triglochin, Ruppia, Lilaea, Aponogeton. 



