84 PHOTOSYNTHESIS AND RESPIRATION. 



The upper and lower skins are thin and transparent, each one 

 cell in thickness ; they are, of course, continuous with each other 

 at the edge of the leaf. In the upper skin all the cells fit closely 

 together without any spaces between them, but in the lower skin there 

 are numerous openings (stomates). Tease the middle tissue with 

 needles, so as to separate the cells in one portion ; notice the shapes of 

 these cells, some being cylindrical, others branched. You will probably 

 find some of the cylindrical cells still attached to the inside of the upper 

 skin, and some of the branched cells to that of the lower skin. 



* (&) Fold a large leaf (e.g. Laurel or Rhododendron) several times, 

 and cut thin sections, keeping the razor wet with alcohol. Mount in 

 water, examine with a microscope, and sketch a thin part. Cut some 

 sections with the razor dry, and mount in water ; notice the presence of 

 numerous irregular air-bubbles between the cells of the spongy tissue, 

 then run in some alcohol and observe the expulsion of most of the air 

 in the form of spherical bubbles. This shows that the spaces between 

 the cells of the spongy tissue contain air. How did this air get in ? 

 In thin sections, try to find stomates leading into the air-spaces. 



* (c) Strip off a piece of the lower epidermis (Broad Bean, Narcissus, 

 Arum, and Ivy-leaved Toadflax answer well), mount in water, and 

 examine with the microscope. Notice the stomates (are they open or 

 closed ?) and the presence of chlorophyll-grains in the guard-cells (do 

 they occur in the other cells of the epidermis ?). Find an open stomate 

 with the high power, put a drop of 3 per cent, salt solution at one side 

 of the cover-glass, and draw it through with blotting-paper. Notice 

 the effect of this on the stomate. Now put a drop of water at one 

 side and draw it through, until the stomate opens again. Sketch the 

 stomate opened and closed. The salt solution draws water out of the 

 guard-cells, and then the stomate closes as the guard-cells lose their 

 turgidity and collapse. When water is added, the guard-cells absorb 

 it and swell up, becoming turgid, and the stomate opens. 



* (d) Cut sections of leaves that have been exposed to light for several 

 hours. Examine some sections to see what parts contain the green 

 grains, then treat others with alcohol and test with iodine. Notice 

 that starch-grains occur only in cells which contain chlorophyll. Tear 

 off bits of the upper and lower epidermis-layers, and notice that only 

 the guard-cells of the stomates contain starch. The starch-grains are 

 very small and are formed inside the chlorophyll-grains. 



A good method is to immerse a thin leaf (e.g. Tropaeolum) in a 

 strong solution of chloral hydrate, made by dissolving a few crystals 

 of this substance in a little water, in a watch-glass, and add a few 

 drops of iodine solution. The leaf soon becomes transparent and the 

 starch-grains stained by the iodine. 



119. The Store of Carbon Dioxide in the Atmosphere. 



Enormous quantities of carbon dioxide are absorbed by 

 green leaves from the air, although the latter usually con- 

 tains not more than 0'03 to 0'04 per cent, of this gas. A 



