242 FIRESIDE SCIENCE. 



which bear true leaves, like the ivy, laurel, or per- 

 haps the holly. We call these plants evergreens ; 

 but in fact they change their leaves as do the de- 

 ciduous varieties. The change is made gradually, 

 one leaf dropping off when another has grown to 

 replace it, and so the tree is never wholly deprived 

 of its foliage. It is probable that in winter there 

 is considerable vegetative activity in these ever- 

 greens, as it is impossible that these changes can 

 take place when the sap is completely dormant. 

 Sunlight and warmth are agents of tremendous 

 power in connection with plant activities. If in the 

 depth of winter a mild day occurs, we shall find, by 

 making incisions in the stem or branches of trees, 

 that the slumbering forces are partially awakened, 

 and the sap is in motion. 



If we allow that in the higher orders of flowering 

 plants winter is a time of repose, it can hardly be 

 supposed that there is no interchange of matter be- 

 tween the air and the body of the plant, for some 

 such movement is needful to its life. The hiber- 

 nating animals in their dens are practically dead, 

 but still a feeble form of life remains; the heart 

 slowly beats, and waste goes on. There is, in fact, 

 a continuous interchange of particles between the 

 air and the body, and so there must be between 

 the air and all plant structures. Men and animals 

 sleep during the night-time, but the functions of 



