Fig. A: leaf; worth noticing is etc.

The word order in English would be slightly different:

hoja donde se destaca una pinna formada por el raquis
-->
hoja *where* una pinna *is in evidence,* formada por el raquis

My option here. From your description, I understand this to be something that "sobresale".

Leaf showing a pinna formed by the

To the point, structure commonly used in captions.

Definition of rachis:
# axis of a compound leaf or compound inflorescence
wordnetweb.princeton.edu/perl/webwn

# division of a usually pinnately divided leaf
wordnetweb.princeton.edu/perl/webwn

Leaves are twice pinnate when the leaflets are attached to a second order rachis. Internal control is the process effected by an entity's board of directors ...
www.ets.org/Media/Research/pdf/RM-04-03.pdf - Similar -

Aralia elata (Araliaceae) image 16447 at PlantSystematics.org
Cultivated; detail of second order rachis and leaflet, adaxially. Copyright © 2008 by Jan De Laet (contact: [email protected]) [ref. DOL16447] ...
www.plantsystematics.org/imgs/jdelaet/.../Araliaceae_Aralia...

2- triplet b, rachis r, to which a first-order stem is attached with two second-order stems of triplet a attached to it, in its turn. ...
www.springerlink.com/index/C4N17301525851L6.pdf

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Note added at 1 day15 mins (2010-02-27 23:28:52 GMT)
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Correction to my answer: the second definition is the definition of a pinna - I forgot to label it, and it appears to be a def. of the rachis, which it is not.