HISTOLOGY OF ANGIOSPEBM STEM, ROOT, ETC. 119 



mesophyll air-spaces by the heat, escape from both sides 

 or only from the lower side. Cut or tear across a leaf before 

 dipping it into the hot water ; note the streams of bubbles 

 issuing from the cut edge. This simple experiment shows 

 that the leaf contains air, and that the air-spaces in the 

 mesophyll communicate with the atmosphere by means 

 of the stomata. 



144. microscopic Examination of Air-spaces in 

 Leaf. Fold a large leaf (e.g. Laurel or Ehododendron) 

 several times, or cut it into strips, and cut transverse 

 sections, keeping the razor dry. Mount in water, and 

 with the microscope note the numerous irregular air- 

 bubbles between the cells of the spongy mesophyll ; then 

 run in some alcohol, and note the expulsion of the air in 

 the form of spherical bubbles. 



145. Tangential (Horizontal) Sections of Leaf. 



Fold a leaf (e.g. Beech, Privet, Laurel, Rhododendron, 

 Ivy) over one finger and, wetting the razor with dilute 

 alcohol, cut thin sections parallel with the upper surface 

 of the leaf; then turn the leaf over and cut sections 

 parallel with the lower surface. 



In each case mount some of the sections with the epi- 

 dermis side upwards, and others with the mesophyll side 

 upwards, so as to have preparations (each mounted in 

 water or dilute glycerine on a separate slide) of (1) upper 

 epidermis, surface view; (2) upper or palisade mesophyll, 

 cells cut transversely and therefore appearing circular with 

 narrow air-spaces at the corners between adjacent cells ; 

 (3) lower or spongy mesophyll, the cells of which appear 

 like starfish, being joined up by their diverging arms so 

 as to form a network, the meshes of which are occupied by 

 air ; (4) the lower epidermis in surface view. Note that 

 the mesophyll- cells contain chloroplasts, and that the 

 vertical walls of the epidermis are usually wavy. 



146. Structure of Petiole. Cut transverse sections of the 

 petiole of various stalked leaves. A stout petiole, if examined by 

 itself, might sometimes be mistaken for a stem, especially when 



