PLANTS MODIFIED BY THEIR SURROUNDINGS 163 



these changes, it becomes evident that the very same factor, water 

 supply, which governs hydrophytic, xerophytic, and mesophjrtic 

 conditions, determines, at least in part, the habits of the plants 

 growing in a given region — be it in the tropics or arctic regions. 

 But in addition to water supply, the factors of temperature, light, 

 soil, wind, etc., all play important parts in determining the form 

 and structure of a plant. 



The effect of wind upon trees in an exposed location. Photograph by W. C. Barbour. 



Cold Regions. ■ — Here plants, which in lowland regions of greater 

 warmth and moisture have a tall form and luxuriant foliage, are 

 stunted and dwarfed ; the leaves are smaller and tend to gather in 

 rosettes, or are otherwise closely placed for warmth and protection. 

 As we climb a mountain we find that the average size of plants 

 decreases as we approach the line of perpetual snow. The largest 

 trees occur at the base of the mountains ; the same species of trees 

 near the summit appear as mere shrubs. Continued cold and high 

 winds are evidently the factors which most influence the slow growth 

 and the size and shape of plants near the mountain tops. Cold, 



