192 STECKBECK— ON COMPARATIVE HISTOLOGY 



are turned toward the sun, so that the incident rays of light fall 

 parallel to the surface of the leaflets. The paraheliotropic movement 

 therefore is above the plane of the horizon instead of below as seen in 

 the nyctitropic response. The sensitive movements of the leaves of 

 Melilotus alba are described by Wilson and Greenman (47), and those 

 of other genera by Macfarlane (31). 



In the sub-tropical regions the members of this family are abundant, 

 with many species of Cassia, Neptunia, Aeschynomene, Desmodium, 

 Melilotus, Acacia, Calliandra, Prosopis, Schrankia, Mimosa, Desmanthus, 

 Caesalpinia, Crotalaria, Indigofera, Sesbania, Arachis, Phaseolus, etc. 

 Practically all of these plants, as far as studied, are sensitive. In some, 

 the sensitivity has been developed to a high degree. The genera that 

 include some of the most sensitive plants of this family are here repre- 

 sented. A number of species of Mimosa, of Schrankia, of Prosopis and 

 of Calliandra appear for the first time. In the tropics numerous species 

 of these genera are found. In the sub-tropics many of the genera com- 

 mon in the temperature zones are also represented here. Some of these 

 extend over 80 degrees of latitude and are found in both hemispheres, 

 as for example, species of Desmodium which extend from south central 

 Canada on the north to sub-tropical Australia and New Zealand on 

 the south. 



In the tropics the Leguminosae reach their climax of development, 

 comprising a large number of genera and species. In the western 

 hemisphere, Brazil is the center of distribution of the family. From 

 this region there have been obtained species of Mimosa, Schrankia, 

 Cassia, Inga, Pithecolobium, Calliandra, Bauhinia, Sweetia, Zollernia, 

 Exostyles, Caesalpinia, Peltogyne, Hymenaea, Parkia, Prosopis, Neptunia, 

 Aeschynomene, Abrus, Vigna, Dolichos and a number of smaller genera. 

 Some genera include a large number of species, — e.g.. Mimosa, more 

 than 300 species; Cassia, over 300 species; Calliandra, nearly 100 species. 



The abundance of leguminous plants in the tropical vegetation, and 

 the very pronounced types of sensitivity exhibited by these, have been 

 noted by many botanists and travelers to the Amazon Valley and other 

 parts of the tropics. De Mello (15, p. 253) writes:— "The order of 

 Leguminosae is here (Campinas, Brazil) a most extensive one, repre- 

 sented by very many genera and innumerable species, from small herbs 

 to the tallest trees. " 



Richard Spruce (44) in "Notes of a Botanist on the Amazon and 

 Andes" refers repeatedly to the abundance and great variety of dif- 

 ferent sensitive plants. He says (p. 257): "But of all orders by far the 



