40 I'LANTS OF NEW ZEALAND 



The surface of tlie leaf is covered by a thin layer of waxy 

 material, which is almost impervious to water. This is 

 called the cuticle, and prevents the leaf from shrivelling up in 

 drying winds. Immediately below the cuticle lies a layer of 

 cells, which form the epidermis. In some cases they are 

 specially modified for the purpose of providing storage 

 Ini' water, as a provision against drought. Below the 

 epidermis of the u[)per surface of the leaf, lies a layer of 

 elongated cylindrical cells, called the palisade cells. These 

 are regularly arranged with their longitudinal axes at right 

 angles to the surface of the leaf, and constitute the wonderful 

 laboratories in which the process of assimilation is carried on. 

 The sap from the roots reaches these cells by way of the 

 midrib and veins. From the outer air they obtain carbon 

 dioxide. They are also provided with a large number of 

 microscopic grains stained with chlorophyll — the cliluroplasts. 

 Jn the chloroplasts, under the action of sunlight — bright or 

 diffused — starch is formed. This is afterwards converted into 

 sugar, and conveyed by means of the couducting tissues or leaf 

 veins, to the parts of the plant where it is required. Below 

 the palisade tissue lie a numljer of loosely arranged cells of 

 I'lirher irregula.r sha[)c, with an spaces between them. These 

 constitute the spoiKjij tissue, and though they doubtless 

 manufacture some food, yet they cannot receive as much 

 light as the cells above them. Consequently they do little work 

 ill the process of assimilation. The ei)iderniis of the leaf is 

 Ijroken in nunrberless places by minute openings, the stomata, 

 wliich have been already mentioned. These are often confined 

 to the lower surface of the leaf. Each stoma is surrounded 

 !)>■ a pair of crescent-shaped guard cells, which by alterations 

 m their form can open or close it, and thus check or 

 increase the ii.niount of evaporation or transpiratioi/ from the 

 leaf. 



Now, although thei'e ai-e thousands of plants m which the 

 leaf structure conforms comparativel\' closely to that alread\- 



