LEAF ARBOREOUS TYPE 101 



2. Vascular bundles of various size, which, in the thinner part 

 of the lamina, lie in a median position between the two epidermal 

 layers. The largest of these correspond in structure to those of 

 the internode, the smaller ones are reduced forms of the same 

 type. Note that the spiral and annular vessels of the protoxy- 

 lem are nearer the upper surface of the leaf. 



Between the epidermis on either side, and the larger bundles, 

 are masses of sclerenchyma, which, together with the bundles, 

 form complete bridges of rigid tissue between the two epidermal 

 layers. 



3. The spaces between the tissues hitherto considered are 

 rilled with parenchyma (mesophyii , which may either be (a) 

 green (containing chlorophyll), or (&) colourless (without chloro- 

 phyll). 



a. The green chlorophyll-containing parenchyma fills up 

 the greater part of the space ; intercellular spaces occur in it. 



b. The colourless parenchyma occurs (i.) as a sheath, without 

 intercellular spaces, surrounding each bundle (endodermis) ; 

 (ii.) as groups of cells immediately below the epidermis : these 

 are more common towards the central part of the leaf. At the 

 midrib this tissue forms the bulk of the structure. 



III. Cut thin tangential sections from the under surface of 

 the lamina, so as to remove, if possible, only the epidermis. 

 Treat with potash, and mount in glycerine. Observe 



1. The ordinary cells of the epidermis, of oblong form, and 

 with sinuous outline. 



2. Short cells between the ends of these, which often 

 project perpendicularly to the surface as hairs, of various 

 form. 



3. The stomata, holding the same position as (2) relatively 

 to the oblong epidermal cells. 



Observe with a high power the structure of the stomata. 

 They consist of 



a. Two narrow guard-cells, which inclose the pore. 



b. Two triangular subsidiary cells, which completely sur- 

 round the convex side of the guard-cells. 



Compare this view of the stoma with the same structure as 

 seen in transverse sections of the lamina. 



