Further Growth and Duration of Plants 31 



branches an undergarment of cork is found. This garment is 

 made of small cells also, but while the cells beneath have 

 spaces between them which admit air, cork cells fit closely 

 together, and the walls are water-proof ; so cork in trees serves 

 the same purpose that it does in bottles. The cork cells are 

 filled with air, and these air spaces, like the loft in a house, 

 assist in modifying the temperature. 



To support and provide food for the branches which are 

 added to the tree each year, the stem must increase in thick- 

 ness. This it does by means of a ring of active cells just out- 

 side the woody portion of the stem. These cells, the cambium, 

 are most active in spring when their walls are so delicate that 

 they are easily broken, and the bark may then be readily re- 

 moved. All the growth outside of this ring of cells is bark. 

 Every boy who makes willow whistles takes advantage of the 

 active season of the cambium, for it is when the cells are young 

 and tender that careful and judicious pounding with the handle 

 of a jack-knife will bring the bark off entire. Later in the 

 season the cambium is not so active (for the whistle-making 

 season is limited, "as every schoolboy knows "), and the cells of 

 the wood are not so large. ^ The large and small cells alternat- 

 ing make the annual rings in the wood by which the age of a 

 tree may be determined. Rings are formed in the bark, but 

 they are not so well marked as those in the wood. 



As the tree grows, the thin green over-dress gets too small ; 

 sap cannot get to it through the cork, so the cells of which it 

 is made starve for lack of food ; it cracks and peels off, and 

 the tree henceforth is clothed in more fitting shades of grey or 

 brown as become its years. Young cork cells stretch, but in 



1 The cause of rings is not fully understood. The former explana- 

 tion of pressure of the old outer cork tissue is no longer held ; nor do they 

 seem to be regulated by the food supply. The oak during some seasons 

 puts forth an earlier and a later growth of branches in one season and 

 thus sometimes forms two rings in one year. These, you notice, occur 

 on the upper branches which have the best exposure to light and air. 

 Down below only one period may occur during the season. Phytolacca 

 dioica,'L., a shade tree from South America and cultivated in this country 

 (Bellombra or Bella sombra) forms twelve well-defined rings in a year's 

 growth. 



