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1370 A TEXTBOOK OF THEORETICAL BOTANY 



(monoporate) or many (polyporate, or if the pori are small, cribellate). 

 In the latter case pori are not limited to the equator. 



The typical dicotyledonous grain would therefore be described as tri- 



colpate and the monocotyledonous grain as monocolpate. Exceptionally, 



larger numbers of colpae occur, up 

 to thirty, corresponding to the 

 number of the lines of contact 

 between polyhedral grains. Acol- 

 pate grains, with no furrow, are 

 also known. 



One or more parallel, spiral 

 furrows occur in a few quite un- 

 related plants, e.g., Eriocaiilon, 

 Mimiilns, Thiinbergm, and in some 

 species of Berheris. 



The pollen grains of the sub- 

 merged marine plant Zostera 

 (Naiadaceae) are filamentous. The 

 *^ mother cells are elongate, the spor- 

 ogenous cells from which they are 

 formed having divided longitudin- 

 ally. A minority divide transversely 

 and they give rise to short cells 

 which are sterile. The mother 

 cells, which measure 5 x 60 

 microns, also divide longitudinally 

 and produce a group of four 



cells which subsequently lengthen until they may reach 2 mm. This 



peculiarity is apparently associated with hydrophilous pollination (Fig. 



1272). 



The pollen of most wind-pollinated plants is smooth and dry, if such 



pollination is characteristic of the whole family, but in genera which are 



exceptional in this respect in their 



families, e.g., Artemisia and Ambrosia in 



the Compositae, the pollen retains the 



family characteristics, though usually in a 



somewhat reduced form. 



The outline of the grain, even in 



radially symmetrical types, shows great 



variation in the ratio of its principal axes 1 2 %\ ^ | § :f g g 2 1 



to one another, ranging from perprolate 



grains, whose polar axis is more than twice 



as long as the equatorial diameter, to 



peroblate grains whose polar axis is less Fig. izv.i-— Pollen shapes, i, Spherical. 



-Zostera marina. 

 pollen grains. 



amentous 



.1 , ,r I • 1 J- rr^i 2, Subprolate. 3, Prolate. 



than halt the equatorial diameter. 1 he prolate. (After Erdtman.) 



4, Per- 



