86 PLANTS 



show a very special response. The usual day-time disposition 

 of the leaves is such as would at night result in the greatest 

 loss of heat by radiation. The leaves of many plants droop at 

 night and thereby come into a position which greatly reduces 

 the loss of heat by radiation. 



Latitude and Altitude 



204. The traveler in passing from the equator to the latitudes 

 of perpetual snow in polar regions observes a gradual change in 

 the character of the vegetation from the most luxuriant evergreen 

 tropical forests to the scanty herbage of those high latitudes 

 where during the few weeks of the brief summer, while the ground 

 is bared of snow, a few specially hardy mosses, a few rapidly 

 maturing annual and biennial herbs and still fewer shrubby 

 perennials succeed in bringing their fruit to maturity. So 

 also in ascending mountain slopes from the sea-level at the 

 equator to the snow line on the higher peaks a similar series 

 of changes in the character of the vegetation occurs with the 

 degrees of altitude. In middle altitudes, as in middle latitudes, 

 there is an intermediate condition of vegetation characterized 

 by the grasses of the prairies and the deciduous perennials and 

 coniferous evergreens of the forests. 



Light 



205. The adaptations of green plants to light conditions 

 has been discussed at considerable length with reference to the 

 disposition of the leaves. It remains to show that the adapta- 

 tion extends also to the formation of palisade tissue and the 

 arrangement of the chloroplasts within the cells of themesophyll. 

 In order to determine whether the palisade tissue is the result 

 of a response to light stimulus, the following experiment was 

 performed. A developing leaf was artificially inverted so that 



