118 HISTOLOGY OP ANGIOSPERM STEM, ROOT, ETC. 



The epidermis of both sides is thin and transparent and 

 consists of a single layer of cells. In the upper epidermis 

 note the closely fitting polygonal cells ; in the lower 

 epidermis the numerous stomata scattered about, each 

 stoma with two curved guard-cells. 



Tease out the middle portion, or crush it under the 

 cover-glass, to separate the tissues. Note the two forms 

 of mesophyll-cells some cylindrical (palisade meso- 

 phyll), others branched in a star-like manner (spongy 

 mesophyll) ; some of the cylindrical cells may be found 

 attached to the inside of the upper epidermis, and some of 

 the branched cells to that of the lower epidermis. Note 

 also the veins, which run out on either side from the 

 median vein in the midrib, and the fine branching veins 

 forming a network; in each vein note the sheath of 

 narrow cells, and the vessels of the xylem (spiral, annular, 

 pitted). 



142. Isolated Epidermis. Remove some leaflets from 

 a Broad Bean seedling. Hold a leaflet, with the lighter 

 green lower side towards you, between forefinger and 

 thumb of each hand and, starting from a point of the 

 edge, tear the leaflet across obliquely. The thin colourless 

 lower epidermis can be torn off in this way, exposing the 

 green inner tissue or mesophyll ; mount the piece of 

 epidermis in water on a slide. Now turn the leaflet over 

 and try to tear off in a similar way a piece of the upper 

 epidermis it does not come off so readily, and more of 

 the green mesophyll is torn off with it, since the mesophyll 

 is relatively compact above and loose below. In this case 

 the epidermis bears stomata on both upper and lower sides 

 of the leaf. 



The epidermis can readily be torn from various other 

 leaves, e.g. Lily, Tulip, Narcissus, Hyacinth, Tropaeolum, 

 Ivy-leaved Toadflax. 



143. Intercellular Air-spaces in the Mesophyll. 



Dip the leaves of various plants into very hot water ; the 

 water should be boiled and immediately poured into warmed 

 tumblers. Note whether the air-bubbles, driven out of the 



