Two Distinct Mimosas

by Bob Harms email-here

Distinguishing M. borealis from M. texana

To create the flower images above individual flowers were separated from the flower clusters, pressed, placed on a scanner, and directly scanned at 1200 dpi for magnification. To view the flower scan sets.

In our immediate area the overall coloration of the plant and shape of the stems and thorns (prickles) is quite different - although these differences are relative in nature. M. borealis has an overall grayish cast; stems are stiff and straight. Older prickles are triangular in shape, gradually flattening to the base, and only minimally curved. Texana stems are darker, new growth brownish-red; stems bend from side to side; and prickles are narrow and strongly curved downward. Flower color is not a reliable distinguisher, but texana is generally much paler.

Barneby and D. Isely 1986 indicates a sharp distinction between M. biuncifera* and M. borealis [p. 119]:

A hypothesis of hybridization between M. biuncifera and sympatric M. borealis A. Gray that might account for observed variation in foliage was rejected on the ground that the latter species is radically different from M. biuncifera in deeply cleft corolla and in stipitate pod breaking up when ripe into free-falling articles, features of which no obvious trace appears in any form of M. biuncifera that might otherwise be a suspected hybrid. Similarities in foliage between M. borealis and M. biuncifera were judged to be more likely due to parallelism than to introgression.

*Our discussion assumes the Barneby and Isely 1986 proposal to split M. biuncifera into two species, one of which is M. texana.

Leaf Structure

Many mimosas (and acacias) are readily distinguished by their leaf structure, by counting pairs of pinnae on a leaf stalk and the pairs of leaflets on a pinna. For our three species, including for comparison two native acacias Acacia roemeriana and A. farnesiana (Huisache), the leaves break down as follows, with values for the largest leaves:

Species	pairs of pinnae	pairs of leaflets
M. borealis (1)	1 to 3 (4)	3 to 7
M. texana (1)	1 to 5 (6)	3 to 9 (10)
M. aculeaticarpa (1)	3 to 8	8-17
A. roemeriana (2)	1 to 3 (4)	5-12
A. farnesiana (2)	2 to 5	9-20

Values from Barnaby 1991.
Values from Isely 1998.

The overlap of pinnae/leaflet values eliminates this factor as a basis for distinguishing our two mimosas (and even A. roemeriana). None of our M. texana exhibit more than 3 pinnae.

Flowers

The bloom consists of a spherical collection of small flowers, pink to pale pink for our native species, white for M. aculeaticarpa. The apparent color reflects the color of stamen filaments and the pistil style rather than the flower's petals/corolla. (Left to right: M. borealis, M. texana, M. aculeaticarpa.)

The difference in flower structure is salient - as with Correll & Johnston 1970, pp. 777-8: "Petals separate to the base or nearly so" for M. borealis. But to observe this requires the aid of a good magnifying glass. Well-preserved pods as well are capable of revealing the distinctive united petals of M. texana.

Individual flowers at near actual size. Click for enlargements.
M. texana	M. borealis

Prickles

The spine of the mimosa is a prickle; i.e., it does not represent a modification of some other structure, such as a leaf or stem, as is commonly the case with many hill-country plants. As defined in The Oxford English Dictionary:

Botanically, a prickle differs from a thorn or spine in that it may be peeled off with the epidermis and does not grow from the wood of the plant

For our plants perhaps the easiest feature for field identification is the position of the prickles between leaf stalks (nodes) on the stem. [Our discussion follows the key given in Barneby 1991, p. 112.] With M. texana (center below) the prevailing pattern is for each prickle to be associated with a leaf stalk (petiole), aligned directly below it. Occasionally there will be a second prickle aligned directly below a stipule* of the same node. With M. borealis (left below) prickles are randomly placed along the stem, and are not necessarily aligned below petioles or stipules, although they may occur there as well. M. aculeaticarpa (right below) presents yet another pattern, with one or two prickles arising immediately below the stipules of a leaf stalk.
* Stipules are the small, short paired appendages on each side of the leaf stalk in the images below; green on M. texana, but already reddish on the others. As the branch ages, these will dry up and fall off.

The leaf stalks have been pruned from the stems pictured,
so that only a stub may be visible.
M.borealis aculei

Pods

The legumes (pods) of M. borealis are said to differ significantly from those of our other mimosas. In general it has fewer pods per cluster, longer and wider pods. Barnaby 1991 indicates that the base of the pod tends to narrow significantly, forming a short stalk (stipe). Other reported differences, such as color, are even more relative and do not seem to provide a clear basis for field differentiation.

M. borealis	M. texana

A more detailed comparison of the pods.

References

Barneby, Rupert, 1991, SENSITIVAE CENSITAE, A Description of the Genus Mimosa Linnaeus (Mimosaceae) in the New World, Memoirs of the New York Botanical Garden 65.

Barneby, Rupert and Duane Isely, 1986, "Reevaluation of Mimosa biuncifera and M. texana (LEGUMINOSAE: MIMOSOIDEAE)," Brittonia 38.119-122.

Correll, Donovan and Marshall Johnston, 1970, Manual of the Vascular Plants of Texas. Renner.

Isely, Duane, 1998, Native and Naturalized Leguminosae (Fabaceae) of the United States: (exclusive of Alaska and Hawaii, Provo.

Turner, B. L., 1959, The Legumes of Texas, Austin.

Plant Resource Center Home Page — Flora of Texas — Mimosa