376 SCOTS PINE, 



(5) The mesophyll, consisting of rather large polygonal 

 cells containing protoplasm, nucleus, and abundant chloro^ 

 plasts and having their cellulose walls infolded to form 

 projections into the cell-cavity; note the peculiar form 

 (like a letter (J in section) of the mesophyll cell or cells 

 below a stoma, the concavity forming the air-space noted 

 above. 



(6) The resin-passages in the mesophyll, each passage 

 lined by a layer of thin- walled epithelium surrounded by 

 a sclereiichyma sheath. 



(7) The bundle-sheath (endodermis) consisting of a 

 single layer of cells. 



(8) The many-layered colourless ground-tissue (peri- 

 cycle) in which are embedded 



(9) The two vascular bundles, each bundle having its 

 xyleiu facing the flat upper side of the leaf and its phloem 

 facing the convex lower side. 



For the detailed structure of the pericycle, unstained 

 sections, or sections stained only with iodine, are usually 

 best. Note that the pericycle consists of two kinds of 

 parenchyma cells, and an irregular band of sclerenchyma 

 which lies below the phloem of each bundle and runs 

 across between the two bundles. The two kinds of paren- 

 chyma cells are (1) cells having cellulose walls and con- 

 taining protoplasm, protein, and starch ("albuminous 

 cells"), and (2) cells having lignified walls with bordered 

 pits, and no contents (" tracheidal cells ") ; the two kinds 

 are termed collectively the transfusion tissue, the former 

 serving for the passage of elaborated organic food from 

 mesophyll to phloem, and the latter for the passage of 

 water and dissolved salts from xylem to mesophyll. 



525. It. S. of Leaf. Cut medium longitudinal sections 



through a foliage leaf, and note the various tissues described above. 

 The mesophyll is usually seen to show a definite arrangement in 

 fan-like groups of cells ; the inner portion (abutting on the endo- 

 dermis) is less compact than that below the hypodermis, air-spaces 

 being present. Note the long lignified cells (fibres) of the hypo- 

 dermis, the resin-duct sheaths, and the sclerotic layer below the 

 bundles ; the tracheids in the xylem of the bundle ; the two kinds 

 of cells in the transfusion tissue. 



