282 EVOLUTION OF PLANTS 
plants (Fig. 59, B). These trees and shrubs, with 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 OF 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 al] 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, 
