ADAPTATION TO CLIMATE. 42 1 



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 intern'o\-en, that the air caught in the inter- 

 stices is not easily displaced, and the leaves are not then subject 

 to the drying effects of the passing winds. 



735. 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 (portulacca) 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. \\'ith 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. 



736. Similarly the common mullein (Yerbascum thapsus), the 

 leaves and stems of which are so densely cohered 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 inter- 

 woven trichomes so as to wet the epidermis. 



737. 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 summits 

 of the northeastern United States, and far northward, has numer- 

 ous 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. 



738. Low stature of alpine plants a protection against wind 

 and cold. — Another protection to plants from winds and from 

 the cold in such regions is their low stature. Many of the her- 

 baceous plants have A-ery short stems, and the leaves lie close to 



