39^ RELATION TO ENVIRONMENT 



pyxidanthera, juniper, etc., come also under this head. In the 

 jumpers the leaves spread out in the summer, while in the winter 

 they are closely overlapped. An interesting example of protective 

 position is to be seen in the case of the leaves of the white pine. 

 During quite cold winter weather the needles are appressed to 

 the stem, and sometimes the trees present a striking appear- 

 ance in contrast with the spreading position of the needles in 

 summer. On windy days in winter, the needles turn with the 

 wind and become rigid in that position so that they remain 

 in a horizontal position for some time, often until the wind 

 dies down, or until milder weather. The following day, should 

 there be a cold strong wind from the opposite direction, the 

 needles again assume a leeward direction. In quiet weather 

 appressed to the stem and in the form of a brush there is less 

 radiation of heat than if they diverged. In strong winds by 

 turning in the leeward direction the wind is not driven between 

 the needle bases and scales. Some plants, especially many 

 of those in arctic and alpine regions, have very short stems and 

 the leaves are developed near the ground, or the rock. Lying 

 close on the ground they do not feel the full force of the drying 

 winds, there is less radiation from them, and the radiation of 

 heat from the ground protects them. Many plants exhibit 

 movement in response to certain stimuli which place them in 

 a position for protection. Some of these examples have been 

 discussed under the head of irritability (see Chapter XIII). The 

 night position of leaves and cotyledons presented by many 

 plants, but especially by many of the Leguminosae, is brought 

 about by the removal of the light stimulus at evening. In many 

 leaves, when the light influence is removed, the influence of 

 growth turns the leaves downward, or the cotyledons of some 

 plants upward. In this vertical position of the leaf-blade there 

 is less radiation of heat during the cool night. The most strik- 

 ing cases of protection movements are seen in the sensitive 

 plant. As we have seen, the leaves of mimosa close in a verti- 

 cal position at midday if the light and heat are too strong. Ex- 

 cessive transpiration is thus prevented. At night the vertical 



