Vegetative Organs of Vascular Plants 745 



greenhouse plant with soft leaves. They are preserved well by FA A 

 and particularly well by Craf II. 



The stereotyped construction of the mesophytic leaf permits the 

 use of innumerable species to illustrate the type, making it possible 

 to utilize plants that are readily accessible and characteristic of the 

 region rather than use some classical species as if it had special virtues. 



Most broad leaves are distinctly dorsiventral, the columnar pali- 

 sade cells being on the upper or ventral side. Leaves that normally 

 assume a vertical position do not have such distinctive palisade cells, 

 the upper and lower tissue zones are nearly alike, and the dorsiven- 

 tralitv is obscured. The garden beet and sugar beet are good examples. 

 These leaves can be fixed successfully in FA A or Craf III. 



The study of the dicotyledonous leaf would be far from complete 

 without a study of deviations from the typical mesophyte. Perhaps 

 the most striking variations are the xerophytic adaptations. The leaves 

 of species of Dianthus show a range from the relatively large, flat 

 leaves of the greenhouse carnation to the waxy, narrow, rolled leaves 

 of the rock garden species. These easily obtainable leaves are well 

 preserved by Craf II. The tough cuticle becomes brittle after xylene 

 but cuts well after TBA or dioxan. A brilliant stain is obtained with 

 safranin-fast green. 



Nereum oleander has leaves of unique xerophytic structure. The 

 lower surface is indented with globose cavities or infoldings of the 

 epidermis. Each pocket is lined with numerous hairs and contains 

 many stomates. The upper epidermis is firm and highly cutinized. 

 There are two to three layers of tough thick-walled hypodermal cells 

 below the epidermis, and the deep-seated j)alisade cells are long and 

 narrow. Killing fluids penetrate with difliiculty. FAA is the most rapid 

 of the satisfactory formulas but may cause slight plasmolysis. If the 

 pieces cut transversely out of the fresh leaf are very narrow, not over 

 1 mm. wide along the linear dimension of the midrib, good penetra- 

 tion and fixation are obtained with Craf 0.30-1.0-5.0. Brittleness in 

 paraffin is minimized by the use of butyl alcohol or dioxan. Safranin- 

 fast green gives a brilliant and highly differential stain. 



Leaves of citrus fruits are also of the leathery type and have an 

 added interesting feature, the pear-shaped oil glands in the epidermis. 

 The spongy parenchyma is compact and firm, and the palisade cells 

 are small and closely spaced. The impervious character of the surface 

 and compactness of the interior necessitate the use of FAA, which 

 produces acceptable results. If the pieces of leaf are cut very narrow, 

 Craf III produces excellent fixation. 



