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Frond
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==Pinnate fronds== {{Main|Pinnation}} In a frond which is pinnate (feather-shaped), each leafy segment of the blade is called a pinna (plural pinnae), the stalk bearing the pinna is termed a petiolule, and the main vein or mid-rib of the pinna is referred to as a costa (plural costae).<ref>{{cite book| last= Walters| first= Keil| year= 1996| title= Vascular Plant Taxonomy| url= https://archive.org/details/vascularplanttax00walt| url-access= registration| edition= 4th |publisher= Kendall Hunt Publishing Co. |location= Dubuque, Iowa}}</ref> If a frond is divided once into pinnae, the frond is called once pinnate. In some fronds the pinnae are further divided into segments, creating a bipinnate frond. The segments into which each pinna are divided are called pinnules, and the extensions of the rachis that support these pinnules, are called rachillae.<ref>{{Cite web|url=http://eunops.org/content/glossary-palm-terms#R|title=Glossary of Palm Terms {{!}} EUNOPS website|website=eunops.org|access-date=2019-12-05}}</ref> Rarely, a frond may even be tripinnate, in which case the pinnule divisions are known as ultimate segments. Pinnae may be arranged along the rachis either directly opposite one another or alternating up the stem. The arrangement may change from the base of a blade to the tip, as in the example of ''[[Blechnum]]'' shown below (from base to tip: pinnae opposite to alternate, and pinnatisect to pinnatifid).
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