124 Our Field and Forest Trees 



There is another movement of sap, which is 

 far better understood. This is called the " mechan- 

 ical movement," and it may begin while snow- 

 drifts are still deep in the fields. Anyone passing 

 through a birch grove, when the sun is shining 

 warmly after an ice storm, may find that drops are 

 falling from boughs which have had their ends 

 snapped off by the wind. 



This dripping of the broken branches is called 

 " bleeding." When the sunset chill comes, icicles 

 form, and they may be seen next morning hanging 

 from the broken boughs. These icicles are never 

 very long, because the " bleeding " does not go 

 on all night. The twilight chill undoes the work 

 of the sunshine, and drives the juices of the tree 

 back into winter quarters. 



Let us see where they go. The little cells which 

 compose the wood of trunk and branches are not 

 fitted together in neat straight rows, like the boxes 

 in a post ofKce. Here and there among them are 

 spaces and crannies. 



In growing time, the cells of the youngest wood 

 are filled, or partly filled, with a living jelly, much 

 like the white of a fresh and raw egg. This is 

 "protoplasm" — a word which means "first 

 jelly." Every living body, whether of plant or 

 animal, is largely built of this substance. 



Plant-protoplasm, in growing time, contains a 

 very large proportion of water. But when cold 

 weather arrives a quantity of this water separates 



