STONECROP. LEAF. 97 



surfaces is recognisable by a different arrangement of the 

 tissues at or beneath those surfaces. It may be noted 

 that the leaf of the Cherry Laurel is of a more pro- 

 nounced bifacial type than that of the Sunflower, since 

 in the latter case stomata arc found on both surfaces, 

 while in the former they occur only on the lower surface. 



Centric Type. 



We have now to study leaves of the centric type, 

 i.e. such as have their tissues arranged symmetrically. 

 It is usually in succulent leaves that this arrangement 

 is found, and they are of an approximately cylindrical 

 form. As an example we may take the leaf of Sedum 

 acre (the common Stonecrop). 



VI. Cut transverse sections of the leaf of the Stone- 

 crop ; 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 uniform all 

 round, so that beginning at any point of the periphery 

 and passing inwards we encounter 



1. The Epidermis, a single layer of cells of variable 

 size and shape, with well-defined Cuticle, and Stomata, 

 the guard cells of which are much smaller than the 

 epidermal cells. 



2. Chlorophyll-containing mesophyll, which is 

 not differentiated into palisade, and spongy parenchyma ; 

 this tissue forms the great mass of the leaf; intercellular 

 spaces occur; the cells are thin-walled, with a proto- 

 plasmic sac, in which are imbedded chlorophyll 

 grains, and there is large central vacuole. Observe the 

 chlorophyll grains undergoing division. Embedded in 

 this tissue lie centrally 



H 



