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A TEXTBOOK OF THEORETICAL BOTANY 



large, echinulate grains, with pori closed by circular plugs of extine which 

 are pushed outwards by the developing pollen tubes (Fig. 1274). The 



Fig. 1274. — Ciiciirbita pepo. Section of a uninucleate pollen 

 grain showing the plugged pori. 



Betulaceae possess characteristic band-like thickenings of the extine, known 

 as arci, which form sweeping curves from pore to pore. The submerged 

 flowers of Naiadaceae and Ceratophyllaceae have smooth-walled grains with 

 no extine. The Onagraceae (Oenotheraceae) have three prominent pores, 

 each covered by an apsis or dome formed principally by the thickening of 





• 



i 



Fig. 1275. — Oenothera biennis. Transverse section of 

 an anther showing the triangular grains with 

 pores at the angles covered by domes. 



the intine. The grains are triangular in polar view and the pores occupy 

 the angles (Fig. 1275). The Cyperaceae, already mentioned on p. 1369, have 



