GENEKAL MOEPHOLOGY OF THE PLANT 53 



mediate portion, the mesopodium or petiole. Either the epi- 

 podium or the mesopodium may be absent. 



The leaf in one or all of these regions generally takes the 

 form of a flattened expansion, owing to the distribution of 

 growth in the phyllopodium leading to the development of a 

 thin wing along two of its sides in a lateral plane. This wing 

 is usually though not always confined to the epipodium, when 

 it gives rise to the leaf-blade or lamina. The branches of the 

 epipodium, when present, are also winged, and the relative 



Fig. 78. 



Fie. 79. 



Fig. 80. 



Fig. 78. Amplexicaul base of tlie leaf in Fool's Parslej'. Fig. 79. Sheath- 

 ing leaf of a Grass. Fig. 80. Decurrent leaf-base of a species of Thistle. 



a. Leaf-base. 



development of the main axis, its branches and their wings 

 causes the various forms of the leaf-blade. 



In the simplest forms of lea.f the phyllopodium does not branch 

 nor show any division into the three regions spoken of. It is then 

 a cylindrical structure as in Pilularia, or a flattened one with 

 no evident wings as in many Monocotyledons, or a winged one 

 as in some of the Gymnosperms. When it branches, the 

 branches usually arise upon the epipodium, and are developed 

 either acropetally or basipetally, according to the position of the 

 growing part. 



In leaves which show a differentiation into the three regions 

 spoken of, each region shows certain peculiarities. 



