LABORATORY PRACTICE 



XIV. Compare with the preceding the arrangement of 

 leaves upon a branch of the same Maple which has grown 

 out in a horizontal direction. Notice that : 



1 . The leaves are also opposite. 



2. They do not decussate, for the internodes are twisted. 



3. The petioles in the different pairs are very unequal ; while 



that of one leaf remains short, that of the other often 

 becomes very long to carry its leaf blade out from 

 under some other leaf blade, in order to obtain its 

 share of light and air. 



4. The effect thus produced (when seen from above) of a 



green surface, made up of leaf blades fitted together, 

 is called a leaf mosaic. 



5. Make a sketch to show this. 



6. Notice the same thing in the phyllotaxy of the horizontal 



branches of other trees. 



XV. Twisting or Torsion of the Internodes has been 

 noticed in one or two cases. This tends to make it difficult 

 to determine the phyllotaxy by bringing the leaves away 

 from their proper positions. It is usually seen best upon 

 horizontal branches of the Forsythia or of young Eucalyptus 

 trees. Notice : 



1. The opposite leaves. 



2. They are arranged very nearly in two ranks. 



3. Each internode, as shown by the lines or angles, is 



twisted through a right angle. 



4. Make a sketch to show this. 



XVI. Sometimes vertical ranks of leaves are made spiral 

 by such torsion of the internodes. 



Examine the Pandanus or Screw Pine of the conservato- 

 ries and notice : 



