THE FOOD OF PLANTS. 



15 



brown ; the covered part loses its white colour only slightly, or 

 not at all, since the chlorophyll pigment absorbs the so-called 

 chemical rays very energetically. 



To determine the depth to which light of an intensity still per- 

 ceptible to the eye penetrates into the tissues, we use, after Sachs, 1 

 the simple diaphanoscope (Fig. 4). This consists of a tube a, 

 60 mm. long, and 35 mm. in diameter, made 

 of stout cardboard. This tube is open at 

 one end, but closed at the other, except for 

 a small opening about 10 mm. in diameter- 

 Over this end of the tube a slips an exactly 

 similar cardboard tube b. If we place the 

 object under investigation between the 

 tubes, hold the open end of a close to the 

 eye, directing the instrument towards the 

 sun or a bright cloud, we can investigate 

 the transparency of plant tissues. I in- 

 serted a piece of Lonicera tatarica leaf in 

 the diaphanoscope ; the light transmitted 

 was bright green. Using four pieces of 

 leaf, it was at once clearly seen that green light passed through. 

 With six pieces, the eye perceived, after looking into the apparatus 

 for some time, a yellowish glimmer. 



To construct an analysing diaphanoscope, it is only necessary to 

 slide a simple diaphanoscope over the front end of the tube of a 

 suitable spectroscope. Daring observation we direct it against 

 bright clouds or the blue sky. I placed a piece of Syringa vulgaris 

 leaf in the diaphanoscope, and found that it permitted the passage 

 of red, orange, yellow, green, and some blue, though certainly 

 diminished in intensity ; the more refrangible rays were completely 

 absorbed. Two pieces of Syringa leaf only transmitted red, orange, 

 yellow, and green very much weakened. A slab of tissue 17 mm. 

 thick from the parenchyma of a potato tuber absorbed the more 

 refrangible rays completely, but transmitted much weakened red, 

 orange, yellow, green, and a trace of blue. According to these 

 observations, then, the less refrangible rays penetrate further into 

 the plant tissues than those of high refrangibility. 



FIG. 4. Diaphanoscope, 

 in longitudinal section. 



1 Cf. Sachs, Sitzungsb. d. Akad. d. IHss zu Wien, 1860, Ed. 43. See also 

 Engehnann, Botan. Zeituny, 1887, p. 393. 



