EXTERNAL CHARACTERISTICS 



211 



with opposite leaves whose arrangement has the precision 

 of a geometrical drawing. As you look down from above 

 you see that each pair of leaves alternates in position with 

 the pair above and with the pair below it ; that is, neigh- 

 boring pairs are set 

 at right angles to 

 each other and with 

 almost perfect regu- 

 larity. Here again 

 the advantage is 

 evident. Members 

 of the mint family, 

 which have square 

 stems as well as op- 

 posite leaves, show 

 this arrangement 

 particularly well. 



Petioles often 

 vary in length on 

 the same plant, and 

 their growth in 

 length appears to 

 be stimulated by 

 lack of light. The 

 result is that they 

 often keep on grow- 

 ing, and the blades as a consequence are kept in the light 

 instead of being overshadowed by the leaves above them. 

 (See Figure 73.) This habit of growth of the petioles is 

 often found in plants with opposite leaves such as have been 

 just described ; the lower down on the stem the leaves are, 

 the longer are their petioles. But it is in connection with 



Fig. 74. — A plant (Cineraria) showing the way in 

 which the points of attachment of the petioles 

 form a spiral around the stem. Note also the 

 differences of the petioles in length. 



