exine

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exine

(ĕk′sēn′, -sīn′)
n.
The outer layer of the wall of a spore or pollen grain.

exine

the outer coat of a POLLEN GRAIN, that often has protuberances, spines, etc. The exine also has one or more areas where the wall is thin, forming a pore through which the pollen tube will protrude.
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Additionally, irregular shaped pollens without extine (Fig.
Also, the formation of irregular shaped pollens without extine is possibly due to the premature disintegration of the middle layer and tapetum.