106 THE NATURE AND WORK OF PLANTS 



flow is due in part to the effect of the sun's rays 

 upon the trunk, heating and expanding the liquids 

 and gases in it, and driving the sap out the auger 

 hole in this way. After the ground thaws out the 

 sap ceases to flow in quantity. 



The sugar obtained from the sap has been stored 

 up in the pith cells in the rays of the wood during 

 the winter. 



142. Dew. — The sparkling gems of dew which 

 make a lawn so beautiful of a summer morning are 

 not usually formed from the air, but are drops of 

 water which have been forced up through the leaves 

 by the root pressure. During the warm and sunny 

 part of the day the water is thrown off by the leaf 

 as fast as it may be received from the roots, but at 

 night the air is cool and moist, and it is not able to 

 do so. As a consequence, the liquid is forced up until 

 it fills the spaces in the leaf and finally oozes out 

 through slits in the epidermis. Of course dew may 

 be formed by the condensation of moisture from the 

 atmosphere, but that on grass usually comes up from 

 the roots. 



143. IToio to cause a plant to form deio at any 

 time. — By putting a plant in the same conditions as 



