136 PRACTICAL BOTANY 



of the mesophyll is a characteristic spongy paren- 

 chyma. 



If tangential sections be cut, it will be found that 

 there are no stomata in the upper epidermis, but large 

 numbers on the lower. 



If transverse sections be cut from one of the upper, 

 vertical leaves, it will be found that below the epidermis 

 of either side there is palisade-parenchyma : stomata 

 are also found in both epidermal layers ; but the 

 vascular bundles retain the position with the xylem 

 directed towards the morphologically upper surface. 

 Note the large lysigenetic oil-cavities. 



Similar oil-cavities are to be found in Ruta, Citrus, &c. Compare 

 also the intra-mural glands in the leaf of Psoralea. Examine 

 young specimens of the above, and trace their development. 



iii. Centric Type. 



The leaves above examined are of distinctly flattened 

 form ; in leaves of another type, called the centric type, 

 the distinction of an upper and lower surface is but 

 very slightly marked, and the whole leaf is of approxi- 

 mately cylindrical form. It is usually in leaves of 

 succulent plants that this arrangement is found, and as 

 an example we may take the leaf of Sedum acre (the 

 common Stonecrop) ; the petiole is entirely absent in 

 this simple form of leaf. 



I. Cut transverse sections of the fresh leaf of the 

 Stonecrop ; mount in water, or dilute glycerine, and 

 observe that the outline of the section is even and 

 oval: the arrangement of tissues is concentric, and is 



