14 ELEMENTARY FORESTRY. 
said to be cither alternate or opposite. When they occur on the 
stump or on roots they are not arranged in any regular order. 
There are two kinds of buds—flower buds, which develop into 
flowers and fruit; and leaf buds, which develop into leaves and 
branches. These can generally be @4istinguished from each 
other by their shape and size and by cutting through them and 
noting their construction. Flower buds are generally more lia- 
ble to injury from climatic changes than leaf buds. 
The Leaves of our trees vary much in size and shape. They 
are simple when composed of but one piece, as the leaves of 
the Oak, Maple and Birch, and compound when composed of 
more than one piece, as the leaves of the Locust, Ash and Black 
Walnut. Leaves are made up of a framework filled in with 
cellular tissue and covered with a thin skin. This skin has very 
many small pores in it called stomata, through which the plant 
takes in carbon dioxide from the air and gives off oxygen and 
water. 
All our trees shed at least a part of their leaves each year. 
All the broad-leaved trees and the Tamarack shed their entire 
foliage yearly, while our so-called evergreen trees lose a part 
of their leaves each year. The length of time leaves remain on 
this latter class of trees varies from two or three years, in the 
case of White Pine growing in very severe locations in this 
section, to perhaps eight years, in the case of Red Cedar favora- 
bly located. The time that leaves remain on the branches of 
evergreens depends to some extent on the location and age of 
the individual tree. 
The following table gives the approximate length of time 
that leaves of conifers remain on trees in Minnesota: 
