SEEDLINGS AND SHOOTS 77 



dovetail together like the pieces of a puzzle. 

 The lovely white bark of the canoe birch is a 

 cork sheet thin and strong as parchment. 



In summer, Nature wisely adjusts the tree's 

 cork garment so that it is not quite air and 

 water tight. This is managed by little vents, 

 or lenticles, in the bark, through which the 

 tree tissues can breathe out superfluous va- 

 pors and gases. The lenticles are little cork 

 cells, round instead of brick-shaped as the ordi- 

 nary cells are, and jumbled together with 

 many a chink and cranny. Not only do the 

 crevices admit the passage of vapor and gases, 

 but air gets through them into the wood. They 

 are often lens-shaped ; hence the name lenticles. 

 It is not easy to find the lenticles on some trees, 

 but they are always there, no matter how thick 

 the bark. We may see them plainly on the 

 trunks of the birch and alder. They stand 

 out as small humps, much lighter in color than 

 the surrounding bark. On the older birch trees 

 the lenticles stretch out horizontally, and may 

 be seen close to the ground as black strips run- 

 ning part way around the trunk. When the 

 tree prepares for winter, little cork seals are 

 built across the vents; for it would be folly to 

 wear a porous union suit in winter. No mois- 

 ture is coming in; therefore none must be al- 



