THE ANGIOSPERMAE 



1369 



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grains in a tetrahedron have each three surfaces of contact with their 

 neighbours. Each contact area develops a median furrow which extends 

 backwards to the distal end of the grain, crossing its equator at right 

 angles. These furrows define the axis of the grain, extending from pole 

 to pole. Once the grains have 

 separated it is very rarely possible 

 to distinguish one pole from the 

 other, but their positions are 

 clearly indicated by the conver- 

 gence of the furrows. Monocoty- 

 ledonous grains have typically only 

 one furrow, along the "deck" of 

 the " boat ", which corresponds to 

 the distal side of the grain while 

 they are still in tetrads. Dicotyle- 

 donous grains are typically three- 

 furrowed. While these are useful 

 distinctions, they are not, as we 

 have said, absolute, for exceptions 

 occur in both groups. For instance, 

 compound grains, due to the fail- 

 ure of the spore tetrads to separate, 

 are found in certain families and 

 genera of both groups, e.g., Epac- 

 ridaceae, Juncaceae, Drimys (and 

 other Winteraceae), Drosera, Typha 

 and Elodea, while linear tetrads 

 are formed in Asclepias and 



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//fl/o/)/z//fl. Some exceptional forms Fig. 1271.— Types of pollen furrowing, i, 



r ■ -iiL ^- J1^ Tricolpate pollen {e.g., Trapa) with 



of grams will be mentioned later. triradiate scar. 2, Three-pored pollen. 



Erdtman distinguishes four 3, Monocolpate pollen. The lower 



, . , r li >> • 11 figures show the above in different aspects. 



kinds of aperture in pollen {After Erdtman.) 



grains. These are not strictly 



speaking apertures, but are the places where the extine may eventually open 



for the protrusion of the pollen tube, or alternatively places where the extine 



is absent (Fig. 1271). 



Sulcus. A longitudinal furrow confined to the distal half of the grain, 

 crossing the polar axis at right angles. 



Colpa. A meridional furrow crossing the equator at right angles and 

 directed at each end to the poles of the grain (see above). 



Ruga. A furrow which is not confined to the meridional region but 

 placed irregularly on the grain surface. 



Porus. A rounded aperture. If present in small numbers pori are 

 confined to the equatorial region. There may be only one porus 



