33$ ECOLOGY. 



from the direct rays of the sun, and also from the fierce blasts 

 of dry air which frequently sweep over these regions. The hairs 

 are so close, and so interwoven, that the air caught in the 

 interstices is not easily displaced, and the leaves are not then 

 subject to the drying effects of the passing winds. 



506. Some plants of temperate regions possess characters of 

 desert vegetation. Even in temperate regions in localities 

 where the climate is more equable, certain plants, strangely, are 

 similarly modified, or provided with protecting armor. The 

 common purslane (portulaca) is an example of a succulent 

 plant, and we know how well it is able to resist periods of 

 drought, even when cut free from the soil. With the oncoming 

 of rains it revives, and starts new growth, while in wet weather 

 cutting it free from its roots scarcely interferes with its growth. 



Similarly the common mullein (Verbascum thapsus), the 

 leaves and stems of which are so densely covered with stellate 

 hairs, is able to resist dry periods. One can see how efficient 

 this panoply of trichomes is by immersing the leaves in water. 

 It is very difficult to remove the air from the interstices of the 

 interwoven trichomes so as to wet the epidermis. 



507. Alpine plants with desert characteristics. Alpine 

 plants (those on high mountains), as well as arctic plants, are 

 similarly modified, having usually either succulent stems and 

 leaves, or small, thick and appressed leaves, or leaves covered 

 with numerous hairs. Cassiope, occurring on mountain sum- 

 mits of the northeastern United States, and far northward, has 

 numerous needle-shaped, closely imbricated leaves. The plants 

 need the protection afforded them by these peculiarities in 

 these alpine and arctic regions because of the dry air and winds, 

 as well as because of the bright sunlight in these regions. 

 Because of the bright sunlight in alpine and arctic regions 

 many of the plants are noted for the brilliant colors of the 

 flowers. 



508. Low stature of alpine plants a protection against 

 wind and cold. Another protection to plants from winds and 



