Violet Wood Sorrell (Oxalis violacea) - 2011-4-8 Muir Woods, CA

found in forest near a trail
Update: ID'd 2016-6-4
some sources show marked leaves and some plain leaves
http://calphotos.berkeley.edu/cgi/img_query?enlarge=0000+0000+0405+1788
http://kentuckynativeplantandwildlife.blogspot.com/2013/03/plant-of-week-violet-woodsorrel-oxalis.html
http://www.illinoiswildflowers.info/prairie/plantx/vlt_sorrelx.htm
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I thought at first this could be Drummond's Woodsorrel but the leaf is quite different and the "V" markings on the leaves are more on the middle of the leaf rather than nearer the stem as in Drummond's.

Drummond's actually flowers pink-ish and is filed way in the "red" album, but since I associate Wood Sorrel with yellow flowers, this is the first place I'd look, so here it is.

Drummond's:
http://www.wildflower.org/gallery/result.php?id_image=12386
Drummond's:
http://plants.usda.gov/java/nameSearch?keywordquery=Oxalis+drummondii&mode=sciname&submit.x=24&submit.y=9
misc oxalis
http://www.pacificbulbsociety.org/pbswiki/index.php/MiscellaneousOxalis
all oxalis at usda.gov - it's not in there
http://plants.usda.gov/java/imageGallery?category=sciname&txtparm=Oxalis&familycategory=all&growthhabit=all&duration=all&origin=all&wetland=all&imagetype=all&artist=all©right=all&location=all&stateSelect=all&cite=all&viewsort=25&sort=sciname