102 



ESSENTIALS OF BOTANY 



veins proceeding from a midrib and thence extending to 

 the margin, as shown in Fig. 66. 



120. Occurrence of Netted or Parallel Veining. — The stu- 

 dent has already, in liis experiments on germination, had 

 an opportunity to observe the difference in mode of vein- 

 ing between the 

 leaves of some 

 dicotyledonous 

 plants and those 

 of monocotyled- 

 onous plants. - 

 This unlikeness is 

 somewhat general 

 throughout these 

 groups of flower- 

 ing plants. What 

 is the difference? 

 The polycoty- 

 ledonous pines, 

 spruces, and othei- 

 coniferous trees 

 have leaves with 

 but a single vein, 

 or two or three 

 parallel ones, but 

 in their case the 

 veining could hardly be other than parallel, since the needle- 

 like leaves are so narrow that no veins of any considerable 

 length could exist except in a position lengthwise of the leaf. 

 The fact that a certain plan of venation is found mainly 

 in plants with a particular mode of germination, of stem 

 structure, and of arrangement of floral parts, is but one 



Fig. 67. The Fall of the Horse-Chestnut Leaf. 



