GROWTH AND SECRETIONS. 67 



which would in trees serve to form ligneous 

 matter, is deposited in their roots, as gum, starch, 

 or sugar, and serves to feed the young shoots of 

 the following year. The newly formed branches 

 of exogenous trees do not grow much in diameter 

 till they have attained their length. 



48. It cannot be said that the ascent of the 

 sap is absolutely null during the winter, but it is 

 then much weaker than in the remainder of the 

 year. In the early spring two phenomena occur ; 

 the heat of the sun begins to be felt on the bark, 

 or cellular envelope, and the more strongly in 

 proportion to the youth of the plant ; the vital 

 action is excited, and the sap begins to rise from 

 the roots, whose spongioles, at this epoch of 

 vernal vegetation, rouse from their lethargic 

 state.* Besides this effect, a second occurs, less 

 visible indeed, but highly important : during the 

 depth of winter, the earth has been warmer than 

 the air ; this comparative warmth is felt by the 

 roots, in which all the accumulated nourishment 



* Perhaps from the circumstance that during the 

 winter the roots being full of the sap, which has been 

 there stored up, are incapable of imbibing more until that 

 begins to rise, which it does as soon as the influence of 

 the sun is felt on the bark. 



