282 EVOLUTION OF PLANTS 



plants (Fig. 59, B). These trees and shrubs, witli all 

 their delicate tissues carefully protected against the 

 effects of severe cold, can endure without harm a tem- 

 perature which would quickly destroy any plant with 

 broad evergreen leaves. 



Most perennial herbaceous plants of cold climates 

 also have special provision against the cold in the de- 

 velopment of underground parts, bulbs, tubers or root- 

 stocks, which remain dormant during the winter and 

 send up their shoots, which grow very rapidly at the 

 expense of the reserve food stored in these subter- 

 ranean reservoirs, so soon as the first warm weather of 

 spring starts them into growth. 



Movements op Plants 



We have seen that the lowest plants are actively 

 motile and how this motility has been retained by the 

 reproductive cells in all but the highest ones. The 

 power of spontaneous movement is common, however, 

 to the protoplasm of all plants, and in the higher plants 

 movements of various organs are sufficiently familiar 

 phenomena. These movements are, to a considerable 

 extent, responses to external stimuli. The bending of 

 growing parts of plants to the light, and the effect of 

 light and temperature upon the opening and closing 

 of many flowers are everyday occurrences. Not so 

 familiar, except to the botanist, are the revolving move- 

 ments of growing tips, especially in twining plants, 

 which are among the most important factors in the 

 twining. Many tendrils are exceedingly sensitive to 

 contact, curving quickly in response to this pressure, 



