STONECROP LEAF 137 



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 embedded chlorophyll- 

 grains, and there is a large central vacuole. Observe 

 the chlorophyll-grains undergoing division. Embedded 

 in this tissue lie centrally 



3. Vascular bundles of small size : their number 

 varies from 3 to 5. 



II. Strip off a piece of epidermis from the leaf of 

 Sedum acre, and mount in water. Note 



1. The nucleated epidermal cells with sinuous 

 outline ; they contain no chlorophyll. 



2. The stomata with two guard-cells surrounding 

 the pore, as in the Holly, or Cherry Laurel. Surrounding 

 these are 



3. Three subsidiary cells, which differ in size and 

 shape from the ordinary epidermal cells, and are 

 arranged in definite order round each stoma. 



Beneath the epidermis will usually be found cells of 

 the mesophyll, with thin walls, large vacuole and 

 protoplasmic sac, in which are embedded chlorophyll- 

 grains. 



By making similar preparations from successively younger 

 leaves the development of the stoma and subsidiary cells may be 



