286 TISSUE ELEMENTS OF SEED PLANTS 



saturated with water vapour, because it is surrounded 

 by living cells saturated with water. The exchange of 

 gases between the living cells in the interior of the 

 plant and the external air takes place by diffusion 

 through this systena of intercellular spaces. 



PRACTICAL WORK. 



(i) Ordinary living parenchyma. Several examples of this, 

 with and without chloroplasts, have already been seen. 



(2) Secretory [gland) cells. Examine demonstration specimens 

 of sections through the enzyme secreting glands on the inner 

 surface of the " pitcher " of the Pitcher Plant [Nepenthes) 

 and also of sections through a nectary. Note in both cases that 

 the cells are densely filled with cytoplasm and have conspicuous 

 nuclei. 



(3) Thick-walled cells, local thickening of cell walls, secretory 

 cells and protein cells in transverse section of leaf of pine (a per- 

 manently stained and mounted section of a large-leaved species, 

 such as P. pinaster, is best). 



Examine first the thick-walled cells in the corner of the leaf. 

 Those just below the surface layer have abundant cytoplasm 

 and conspicuous nucleus — ^they are stiU active. Note the 

 stratification of thq walls, the middle lamella, and the pits. 

 The cavities of the epidermal cells are almost obliterated. Note 

 the thick layer of cuticle on their outer surfaces. 



The mesophyll (photosynthetic) cells below the thick-walled ' 

 tissue have numerous choloroplasts and plate-like projections 

 of the walls (local thickenings) into the cavities. 



Note the resin canals at intervals (one in each corner of the 

 leaf). These are intercellular channels surrounded by secretory 

 cells, and (outside these) thick-walled cells. The resin is expelled by 

 the secretory cells and accumulates in the canal. 



Examine the protein cells on the outer flanks of the central 

 pair of bundles. These are rich in cytoplasm and have conspicuous 

 nuclei. They adj oin the sieve tubes of the bundles. 



Note also the lens-shaped bordered pits on the walls of some 

 of the cells around the bundles. 



(4) Stone cells. Tease out on a slide in a drop of dilute glycerine 

 a little of the flesh of a pear, and note the thick-walled pitted 

 stone cells, singly and in groups, among the thin-walled paren- 

 chyma. When the stone cells are numerous the pear is gritty 

 between the teeth. 



