THE MIMOSA FAMILY 



MIMOSACE^ Reichenbach 



i|HIS family comprises some 35 genera, with about 1350 species of herbs, 

 shrubs or trees, which are common in the warmer portions of the 

 world, but are most abimdant in the tropics, where some of them are 

 of great economic importance, the well-known gum arabic being a 

 gummy exudation of various species of the genus Acacia. Some of the most 

 important shade trees of the tropics, also, are members of this family. The leaves 

 of many of these plants are very sensitive, responding, by drooping, to the shghtest 

 touch, even a slight disturbance of the surrounding atmosphere being sufficient 

 to produce this effect, as in the case of the well-known Sensitive plant. Mimosa 

 pudica Linnaeus, of our greenhouses, which is a conunon weed in the tropics. 



The leaves are alternate, mostly compound or 2 to 3 times evenly pinnate, stalked 

 and stipulate, the stipules often persistent and spine-like. The flowers are small, 

 mostly in spikes or heads, usually perfect, sometimes polygamous; calyx 3-to 6- 

 lobed or toothed, the lobes valvate in the bud; petals equaling the sepals in num- 

 ber, distinct or partly imited; the stamens equal the petals in number, or are 

 twice as many, or very numerous, usually conspicuously much longer than the 

 corolla, their filaments distinct or imited at the base; pistil a single carpel; ovary 

 superior, i -celled; style simple, tipped by the small stigma. The fruit is a legu- 

 minous pod, variously shaped and often contorted; the seeds are with or with- 

 out endosperm; cotyledons thick and fleshy. 



In addition to the trees here described about 60 species of shrubs and herbs 

 in 8 genera occur in the United States. 



Our genera, with arborescent species, are: 



Stamens more than 10. 

 Filaments partly united into a tube. 

 Pod splitting into halves through the thickened margin. 

 Pod thick, leathery, contorted; flowers in globose heads; leaves few- 



foliolate. 

 Pod hard, woody, straight or nearly so; flowers in spikes; leaves 



many-foliolate. 

 Pod thin, leathery. 

 Flower heads small, 1.5 to 2 cm. in diameter; ovary stalked; leaf- 

 lets nearly symmetrical. 

 Flower heads large, 3 to 4 cm. in diameter; ovary sessile; leaflets 

 one-sided. 

 Pod not splitting into halves through the margin, but breaking away 

 from it. 



S13 



I. Pithecolobium. 



2. Siderocarpos. 



3. Havardia. 



4. Albizzia. 



5. Lysiloma. 



