43 



4. Secondary roots — growing from slips or 

 stems, as in willow. 



(b) Stems and Branches. 



2. Tree — plant of woody structure branching 

 some distance above ground. 



2. Shrub — ^plant of woody srucure branching 



directly above ground. 



3. Exogenous stems — separable bark, wood in 



annual layers, as maple, oak, etc. 



4. Endogenous stems — no separable bark. 



Woody substance in threads within pithy 

 material; as palmetto, cornstalk, etc. 



(c) Bark. 



1. Birch — bark peels in thin horizontal layers. 



2. Ash — ^bark opens in many irregular netted 



cracks near each other. 



3. Chestnut — bark opens in longitudinal 



cracks quite distant from each other. 



(d) Wood. 



1. Heart-wood-^dead, dark, central wood. 



2. Sap-wood — carries sap in growing season. 



3. Medullary rays — silver grain. 



4. Annual layers — minute tubes or cells. 



Large in early growing season; small in 

 late growing season. 



5. Age of tree generally told by annual layers. 



(e) Branches. 



1. Opposite leaves, generally opposite 



branches. 



2. Alternate leaves, always alternate 



branches. 



3. Erect, horizontal and drooping Lombardy 



poplar, pine and weeping willow. 



