The Science of Plant Growing 



43 



into three or five main branches at the base of the blade. The spaces 

 between the veins are occupied by thin - walled cells of various forms 

 according to their function. A section 

 through a part of this kind of tissue 

 in Franciscea (or Brunfelsia) is shown 

 in fig. 29. The layer of empty cells on 

 the upper side is the skin or epidermis, 

 the cells of which are filled with water 

 in the live state. The outer walls on 

 the exposed surface are more or less 

 thickened, the thickening being termed 

 the cuticle. In leathery leaves, like 

 those of a Camellia, Palm, Laurel- 

 cherry, or Rhododendron, the cuticle 

 is considerably thickened to keep out water and to prevent the loss of it 

 from within. In many leaves the interior (fig. 31) of the wall is greatly 



Fig. 30. Vertical section through a Leaf, show- 

 ing Epidermis, Palisade, Spongy Tissue, and a 

 Stoma cut through 



Fig. 31. 1, Shows Epiderm with thickened upper wall and palisade cells beneath, filled with 

 chlorophyll. 2, Epidermal Cells thickened on one side, with Cellular Tissue beneath. 



thickened to impart strength and protection to the softer tissues. Below 



the skin are palisade cells, placed at 



right angles to the surface and one, 



two, or more layers deep. These are 



filled with protoplasm, chlorophyll, 



starch, sugar, and sometimes other 



substances. Between the palisade 



cells and lower epidermis lies a mass 



of spongy tissue not always so open 



as in this particular leaf, but filled 



more or less with similar materials 



to the palisade cells. The lower 



skin shows two air pores or stomata 



cut through and leading into breath- Fig. 3-2. stomata 



ing or respiratory cavities. The latter 



are continuous with others leading 



all through the leaf. The air pores 



of land plants are most often situated on the lower surface of the leaf, but 



may be equally numerous on both sides of the leaves of Iris, Carnation, 



Surface view of a portion of the Frond of a Fern, 

 Nephrodium FUix-mas. 



