82 PHOTOSYNTHESIS AND RESPIRATION. 



118. Structure of the Leaf. A fuller knowledge of the 

 structure of the leaf will help us to understand its functions 

 better. Even if you have no microscope, you can learn a 

 good deal about leaf -structure by simple methods. 



The leaf (Fig. 36) is covered externally by a layer of 

 flattened cells (skin or epidermis'), and the outer walls of 

 these skin-cells are chemically changed, the cellulose being 

 cutinised by the addition of a substance (cutin) which is 

 almost impermeable to water and gases. This substance, 



OLAMD-HAIR-^ 

 UPPER 

 EPIDERMIS-; 



WOOD- VESSELS 



BAST 



-SHEATH 



Fig. 36. Structure of Part of a Broad Bean Leaflet. 



which resembles cork in some respects, is most abundant in 

 the outer part of the cell- wall, so that the epidermis is covered 

 by an outer sheet or cuticle. The cuticle acts as a protection 

 against drying ; it is especially well developed in the leaves 

 of plants that grow in exposed situations and in those of 

 "evergreen" plants, while it remains undeveloped in plants 

 which grow submerged in water. 



The epidermis is discontinuous at certain points, leaving 

 narrow oval apertures called stomates, each of which is 

 surrounded by a pair of special curved guard-cells. The 

 latter are able to open or to close the stomates, for when 

 they are quite tense (turgid) they curve apart, but when less 

 so they collapse and come together. The stomates are very 



