from the July 21, 2013 Newsletter issued from the Frio Canyon Nature Education Center in the valley of the Dry Frio River in northern Uvalde County, southwestern Texas, on the southern border of the Edwards Plateau; elevation ~1750m (~5750 ft); N29.62°, W99.86°; USA
SWALLOW-WORT
Occasionally you find dense tangles of a delicate-looking, small, wiry-stemmed vine bearing two slender leaves per stem node, the leaves being only about half an inch long (15mm), and often the vine is flowering profusely with tiny, white, star-shaped blossoms about the size of a mouse's eye, as shown above.

Above, a close-up of two flowers shows that from the sinuses between the five petals' bases, slender, sharp-pointed, fingerlike appendages arise, which don't appear in most other kinds of flowers. Also, in the flowers' centers, instead of the usual oval ovary topped by a slender style and pollen-grabbing stigma, there's some kind of thick, round-headed structure.

This week the vines' first pod-type fruits began splitting open, releasing white-parachuted seeds into the wind. An empty pod and a pod about to open are shown below:

While examining the vine, if you accidentally tear a leaf, the wound bleeds white latex, and that's an important field mark. Along with the flower type, opposite leaves, pod with parachuted seeds, the milky latex points us to the old Milkweed and Dogbane Families. The question of which of those two families our vine belongs to is moot because the Milkweed Family has been sunk into the Dogbane Family.

So, here we have a member of what used to be the Milkweed Family, but now is the Dogbane Family, the Apocynaceae. It's the MacCart's Swallow-wort, CYNANCHUM MACCARTII*, a rather uncommon little vine occurring only in southwestern Texas and adjacent northeastern Mexico. It's common here, just not found in other places.

The name swallow-wort is applied to several species in different families.

I'll bet that in the old days this vine was used medicinally, because members of the Dogbane Family often contain very powerful alkaloids in their milky sap, plus nearly always plants oozing white latex are regarded as medicinal by various cultures.