)
)
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.
| 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 |
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.
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.
 M. texana |
 M. borealis |
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.
| M. borealis | M. texana |
|---|---|
 |  |
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.