385 



CHAPTER VI. 



THE STRUCTURE OF THE LEAF. 



The leaf, or phyllopodium, arises as a little outgrowth from the 

 stem in close proximity to the growing point ijirj. 770, I). It 

 generally follows in its method of origination the mode of for- 

 mation of the meristem of the axis ; if the latter has an apical 

 cell, the leaf is similarly constructed ; if there is a small- 

 celled meristem in the stem, 



FiCt. 770. 



pe pi 



the leaf also has no apical 

 cell. The tissue of the leaf 

 arises from the dermatogen 

 and the periblem, both of 

 which produce a number of 

 cells causing the projection. 

 In further development the 

 dermatogen gives rise to the 

 epidermis, and the periblem 

 to the internal tissue, the 

 plerome of the stem taking 

 no part in its formation. 



Like the stem the phyllo- 

 podium may be monostelic 

 or polystehc. In either case 

 the meristeles or separate 

 steles of the leaf become 



united with the stele or 



, J. ,, •■LI. Fi'^l- '^^>- Growing point of the stem of a 



Steles Ot the axis by changes bicutyleilon. After Douliot. rf. Dermatogen. 



in the cells of the periblem P'- ]?fiw=™- ^'- P'™™""- '■ Young leaf. 

 \ The thick lines are exaggerated to show 



which lie between the axial the limits of the three regions. 



stele and the insertion of the 



leaf. The endodermis and the pericycle of the two members be- 

 come similarly continuous with each other. As the leaf grows, 

 it assumes the shapes already described, by variations in the 

 distribution of growth, part or all of the projection becoming 

 flattened or winged. Usually the three regions described as 



VOL. I. CO 



