88 PROPAGATION OF PLANTS. 



must be made good from the parts below. Whenever 

 there is a tojbal cessation of the movement of fluids 

 through the stems and branches death follows, and in 

 ligneous plants the wood, bark and buds become dry and 

 shriveled. We may, therefore, conclude that the entire 

 sap of trees never becomes frozen solid, and that there is 

 always a flow of gaseous matter, if not of heavier liquids, 

 through the cells, even when the plants are in a semi- 

 dormant state. The often repeated experiment of forcing 

 into growth under glass a cane of a Grapevine or branch 

 of a fruit tree while attached to the parent plant, remain- 

 ing out of doors and apparently frozen, shows that there 

 must be some communication between the semi-dormant 

 parts and those within the house. 



The first effect of light and warmth in spring is to 

 stimulate action in the plants. The fluids absorbed from 

 the soil by the roots are carried upward from cell to cell, 

 through the alburnum or sap-wood of exogenous stems, 

 to the leaves and buds, where they are exposed to air and 

 Jight, and there changgd into organizable mattei^hrgugh 

 a process which is termed assimilation, Some of the 

 liquid pa<rt of the sap is exhaled, passing off into the 

 atmosphere, while a portion of the assimilated matter 

 goes to aid in the prolongation of the twigs, enlargement 

 of the leaves, buds, flowers and fruit, and other portions 

 are spread over the entire surface of the plant through 

 the liber or inner bark, even extending down to the ex- 

 tremities of the roots, adding to their size and prolonga- 

 tion. In this way the concentric layers of wood are 

 formed on the outside of the stem and branches. If 

 there is no cessation of growth during the summer the 

 newly-formed cells coalesce, producing a homogeneous, 

 uniform concentric layer which may be readily distin- 

 guished from those of previous years. This is the usual 

 method of growth in exogens, but the sap may be diverted 

 from its natural course, lor if obstructed, the tissues will 



