PASSAGE OF LIGHT THROUGH LEAVES. 309 



821. The depth to which light can penetrate green tissues. This 

 can be ascertained approximately by a simple apparatus sug- 

 gested by Sachs. 1 A pasteboard tube, a foot or so in length 

 and about an inch in diameter, is cut at one end so as to fit 

 around the eye very closely and allow no rays to enter except 

 through the other end of the tube. If a thin leaf be placed 

 over the distal end of the tube, and it be held towards a bright 

 light, a large portion of the light will be received by the eye. 

 If leaf after leaf be placed over the first, the green color soon 

 gives way to a dull red, and finally is excluded altogether. 

 The same apparatus shows to what depth light can penetrate 

 superposed layers of green cells taken from a stem or from thick 

 leaves. 2 



822. The quality of the light which penetrates a leaf, or which 

 has passed through one layer of cells containing chlorophyll, is 

 shown b} r means of the spectroscope. From what has been 

 shown (p. 296) , it is clear that the light which acts on the cells 

 below the first layer exposed to the sun's rays must be different 

 from the incident rays themselves. The light which reaches 

 the deeper tissues of a leaf has passed through more than one 

 film of green tissue. 



823. The degree of intensity of white (that is, uncolored) light 

 most favorable to assimilation has not been determined with 

 certainty. The lowest limit at which any assimilation has been 

 observed is considerably above that at which etiolated chloro- 

 phyll turns green. 8 



824. It has been shown 4 that very intense white light, even 

 after it has been deprived of nearly all of its heat rays, can 

 destroy the vitality of vegetable cells. Considerably before the 

 death of the cells from this cause, the chlorophyll granules in 

 them lose all their coloring-matter, even when they preserve 

 their general form, and having once lost their green color, do 

 not afterwards regain it. 



1 Handbuch der Experimental-physiologie, 1865, p. 5. 



2 But it has been shown by Haukel that the angle at which a beam of light 

 strikes a plate of glass makes a noticeable difference in the amount of the chemi- 

 cal rays which can pass through it ; thus while at a vertical angle 81 per cent 

 of the rays are transmitted, the rest being absorbed, at an angle of 60 the 

 amount transmitted is reduced to 71 per cent, and at 80 to 33 per cent. The 

 subject as relating to plants has not received the attention it deserves (Berichte 

 iiber die Verhandlungen der Sachsischen Gesellschaft der Wissenschaften). 



3 Sachs : Experimental-physiologic, 1865, p. 8. 



* Pringsheim : Monatsberichte der Berlin Akademie, 1879. 



