PHOTOSYNTHESIS 89 



screen attached, is brought on the table beside the blanching apparatus 

 (of Fig. 14). The leaf is removed, immersed in the hot water, kept under 

 alcohol in the test-tube until white, covered with water for a minute or two, 

 placed in a flat glass vessel standing on a white dish or tile, and covered with 

 iodine solution; then the pattern should soon appear. It may be passed thus 

 around the class, or the leaf may be removed, placed on the white tile, and 

 held up for inspection. A very striking result is given by use of a photo- 

 graphic negative as a screen. It is possible to preserve the leaves perma- 

 nently in 50% alcohol, and although the pattern fades out, it can be restored 

 at any time by fresh application of iodine. 



Dark Chambers. When plants are to be kept for a day or more in 

 darkness, the conditions should be in every respect as nearly normal as pos- 

 sible. Best of all for this purpose, and most convenient, is a ventilated dark 

 room, built into and taking the temperature and moisture of the greenhouse 

 itself, as described earlier in this book (page 34). Next in value is a venti- 

 lated dark box, upon a similar principle (page 43). After this in efficiency 

 would come the normal small chamber later described under Growth; it 

 is readily adaptable to this purpose. Improvised arrangements should ensure 

 healthful conditions of temperature, moisture, and purity of air. 



Iodine Tests for Starch. One of the most valuable and striking of 

 all tests for organic substances in the plant is the blue color imparted to 

 starch by contact with iodine in solution, due to the formation of the blue 

 iodide of starch. The test has the advantage that it may be used either 

 upon a microscopical or a macroscopical scale, its striking advantages for 

 demonstration having first been shown by Sachs in 1884 (Arbeiten des 

 botanischen Instituts in Wiirzburg, 3, 1884, 1, and his Gesammelte Abhand- 

 lungen, 1, 1892, 354). Iodine (I) is a flaky-crystalline, black-violet lustrous 

 solid, brown in solution, very sparingly soluble in water (about 1 g. in 5200 cc), 

 but dissolving readily in water containing alcohol or potassium iodide (KI). 

 The latter is the most useful and inexpensive solution for use on a large 

 scale; a good strength is, — iodine 1 gram, potassium iodide 5 grams, water 

 1 liter (most quickly made by placing the potassium iodide in about 50 cc. 

 of water, adding the iodine, and, when all is dissolved, pouring the solution 

 into the remaining water). The tincture of iodine of the druggists, diluted 

 with about 20 times its bulk of water, is also good. These strengths are 

 much weaker than commonly recommended, but, in my experience, are 

 amply strong. Very much stronger solutions are, however, advantageous 

 for microscopical use, and Strasbtjrger recommends iodine 5 eg., potassium 

 iodide 20 eg., water 15 cc. Iodine, as well solid as in solutions, and even 

 in some combinations, is readily volatile, and hence can only be preserved 

 in closely stoppered bottles. 



At times iodine does not immediately stain the starch of living cells, 

 because of an enveloping film of living protoplasm. Immersion in hot water, 

 however, will kill the protoplasm, swell the grains, and show the reaction 

 at once. 



Erroneous Experiment. One of the most familiar experiments of 

 the text-books is that in which two corks are pinned matching on the two 



