194 STECKBECK— ON COMPARATIVE HISTOLOGY 



response to the three methods of stimulation given are found in the 

 tropics. The forms with the most highly specialized sensitiveness to 

 the greatest number of stimuli are natives of the tropics, especially 

 in the New World. 



Mimosa pudica, the best known of the sensitive plants, is native 

 to the Amazon Valley, but has been introduced into Asia and Africa, 

 so that sensitive plants now abound in many parts of the eastern and 

 western tropics. Mimosa Spegazzini, which is more sensitive than 

 M. pudica, is also indigenous to the South American tropics. These 

 two forms represent the acme of leaf sensitivity among the forms that 

 are in cultivation and that have been studied. There may be others 

 in the tropics that are equally sensitive, or even more sensitive, but which 

 have not been studied. 



Why the climax in the number of species of sensitives and in the 

 high degree of irrito-contractile movements in the tropics? This can 

 probably be attributed to the climatic conditions peculiar to the tropics. 

 During the day, the temperature averages approximately 35° C. The 

 atmosphere is rather humid with rainfall every afternoon during the 

 wet season. 



Bates (1) gives a very interesting account of the equatorial climate. 

 He refers to the shortness of the twilight and the consequent rapid 

 transition from day to night and from night to day. The deposition 

 of dew on the foliage at night is very heavy. The nights are cool. In 

 describing the daily rains he writes: "The heat increased hourly, and 

 toward two o'clock reached 93° F. The leaves which were so moist 

 and fresh in early morning, now became lax and drooping. On most 

 days in June and July a heavy shower would fall some time in the 

 afternoon. The cool sea breezes died away. The heat and the electric 

 tension of the atmosphere would then become almost insupportable. 

 The whole eastern horizon would become almost suddenly black. Then 

 the rush of a mighty wind is heard through the forest, swaying the tree 

 tops; a vivid flash of lightning bursts forth, then a crash of thunder, 

 and down streams the deluging rain. After the rain heaps of flower 

 petals and fallen leaves are seen under the trees." 



The high temperatures by day, the sudden drop of temperature after 

 sunset, the rapid transition from day to night, the heavy dew, the 

 heavy daily rains with the accompanying winds, and dark skies (and 

 the presence of browsing animals, as suggested by Wallace), all these 

 factors aid in explaining the advantage resulting from closure of the 



