Prostrate Sandmat

Euphorbia prostrata

Description 4

Plants prostrate annuals (may overwinter in cracks in sidewalk), ascending in shade or competition, hairy, typically not more than 15 cm tall and usually forming mats up to 20 cm. Largest internodes up to 2 mm in diameter. Stipules divided. Leaves oval to rotund, shallowly serrated. Glands and appendages usually pinkish. Fruits and ovaries with pilose hairs. Seeds with abrupt, sharp ridges along the surface, seeds white.

Comments 4

Plants are most similar to E. stictospora in Texas. It can be distinguished from it by the following characters:

Euphorbia prostrata: Appendages a light purplish pink with magenta gland; hair on capsules usually not dense and straight (pilose or occasionally curving or even nearly appressed); seeds ridged, not pitted.
Euphorbia stictospora: Appendages usually white with yellow or sometimes red glands; hair on capsules dense and curving (villous); seeds pitted.

When reading descriptions like the one in Manual of the Vascular Plants of Texas (Correll and Johnston, 1970), it can be difficult to distinguish E. prostrata from E. maculata without looking at the seeds or hairs. The leaves are also quite different and can provide a good character to differentiate the two, especially in combination with the other characters:

Euphorbia prostrata: Appendages characteristically light purplish pink, as long as the gland is wide or shorter, all of equal or subequal width; glands magenta and smaller than those of E. maculata; cyathium peduncle as long or longer than the cyathium; oval to rotund leaves always lacking a "splotch"; leaf bases more equal than E. maculata (still unequal); hairs on fruits generally not appressed, if appressed, absent between the keels or at least sparser than the density on the keels; ridges on seeds coming to a sharp edge.
Euphorbia maculata: Appendages characteristically white (pink under stress), as long or longer than the gland is wide, those subtending proximal glands larger than those subtending distal glands; glands yellow to magenta and larger than those of E. prostrata; cyathium peduncle shorter than the cyathium; oblong leaves often with a reddish or purplish “splotch” on them (not always); leaf bases more unequal than E. prostrata; hairs on fruits always appressed and uniformly distributed; ridges on seeds rounded.

Because of the abundance of these two species as lawn and garden weeds, it is usually fairly easy to find the two species to compare.

During the 1930's and 1940's, Louis Cutter Wheeler (the Euphorbia expert of the time) considered this species a synonym of E. chamaesyce. This has long since been resolved with the name E. prostrata accepted as distinct from the strictly Old World E. chamaesyce. Occasional specimens representing this outdated taxonomy can be found. On iNaturalist, the two share the same common name which has occasionally led to confusion. The above information combined with the following morphological characters easily distinguish the two species.
Euphorbia prostrata: fruit hairs and stem hairs different; fruit hairs straight, pilose (occasionally somewhat appressed), and mostly restricted to the keels of the fruits; stem hairs short and crinkled; leaf bases closer to equal; plants native to New World, introduced to Old World.
Euphorbia chamaesyce: fruit an stem hairs essentially the same type; fruit hairs straight, pilose, and uniformly distributed throughout the fruit; stem hairs straight, pilose; leaf bases strongly unequal; plants restricted to Old World.

Geographic trends: In Arizona, some populations of Euphorbia prostrata have entire leaves.

Sources and Credits

  1. (c) Linda Jo, some rights reserved (CC BY-NC), uploaded by Linda Jo Conn, http://www.inaturalist.org/photos/1374136
  2. (c) ellen hildebrandt, some rights reserved (CC BY-NC), http://www.inaturalist.org/photos/4027379
  3. (c) nathantaylor, some rights reserved (CC BY-NC-SA), uploaded by Nathan Taylor, http://www.inaturalist.org/photos/5167482
  4. (c) Nathan Taylor, some rights reserved (CC BY-SA), http://www.inaturalist.org/guide_taxa/346779

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