THE STEUGTURE OF THE LEAF 



389 



contains bands of collenchyma or solerenohyma these are found 

 in the midrib also. The vascular bundles send out branches 

 into the wings, the distribution of which varies with the dimen- 

 sions of the latter. 



If the epipodium is branched, the branches may remain 

 distinct or their wings may be fused together to a very variable 

 extent. The secondary axes are generally distinguishable as 

 ribs much like the midrib, and possess a similar structure. If 

 the branching is cymose, there is no main midrib, but several 

 spring from the junction of petiole and blade. As before, these 

 have a very similar structure to that of the petiole. 



As the termination of the main axis or any of its branches 

 is approached, the axial character becomes less and less con- 

 spicuous, till it is indis- 



FiG. 773. 



tinguishable froixi that 

 of the wings with which 

 it is continuous. Where 

 the fusion of the wings 

 has taken place, the 

 vascular bundles from 

 the different axes fre- 

 quently anastomose with 

 each other, forming a 

 complicated network in 

 which the bundles ter- 

 minate either separately 

 or in the form of a 

 meshwork. 



The transition from 

 the structure of the 

 petiole is thus a gra- 

 dual one, the successive 

 axes becoming more and 

 more flattened till they 

 lose altogether their 

 cylindrical character. 

 The epidermis is conti- 

 nuous over the whole 

 surface, and is not much 

 altered ; the cortex pecu- 

 liar to the axis becomes gradually replaced by that peculiar to 

 the flattened wings ; the vascular elements are gradually 

 attenuated till only a few tracheids remain conjoined with a 



Fig, 773. Section of midrib of leaf of PHinula 

 ejp. Epidermis, co. Cortex, en. Endodermis 

 pe. Pericycle. pli. Phloem, a. Xylem. 



