294 ASSIMILATION. 



780. The fluorescence of chlorophyll pigment is best shown by 

 allowing rays of light, made convergent by passing through a 

 double convex lens, to fall upon the surface or side of a strong 

 alcoholic solution of chlorophyll. The color at the focus of the 

 lens will then appear blood-red, but by transmitted light the same 

 solution will appear dark green. By fluorescence is meant the 

 property possessed by certain substances of diminishing the re- 

 frangibility of some rays of light ; in the case of chlorophyll all 

 the rays towards the violet end of the spectrum are made to 

 conform in refrangibility to those near the red. A bright solar 

 spectrum l cast upon the side of a flat vessel containing a solu- 

 tion of chlorophyll appears much like a stripe of dull red : in 

 this red stripe are bands corresponding in their position to the 

 absorption-bands of chlorophyll. If the blood-red color produced 

 by a strong light falling on the surface of a concentrated solu- 

 tion of chlorophyll is examined through a spectroscope, only 

 red rays having the same degree of refrangibility as those of 

 the deep absorption-band of the chlorophyll spectrum come to 

 the eye. 



781. Plants without chlorophyll. If whole plants (certain 

 parasites and saprophytes, for example, Monotropa) are either 

 white or slightly tawny throughout, it is owing to a complete or 

 partial absence of chlorophyll ; but in some instances such plants 

 may impart to alcohol, in which they are immersed, a decided 

 tinge, frequently blue. 



782. " Colored" plants. When leaves or stems have some 

 color other than green, they are said to be colored ; if two or 

 more different colors are intermingled the parts are variegated. 



783. In the case of healthy leaves exposed to light, white 

 spots, streaks, etc., are generally, if not always, characterized 

 by an absence of chlorophyll. Such spots have relations to their 

 surroundings which are different from those of the contiguous 

 green parts ; the)- do not have the power of assimilating in- 

 organic matters. 



784. In plants, the paleness of colors verging upon green or 

 blue (for example, those in many kinds of cabbage) sometimes 

 depends wholly on the existence upon the surface of the part, of a 

 great amount of the waxy matters known collectively as bloom 

 (see 226). The tissues beneath the surface may be vivid green. 



785. Red and yellow colors of healthy and vigorous leaves are 

 usually due to the presence in the cells (often merely those of 



i Hagenbach: Annalen der Physik und Chemie, cxlL, 1870, p. 245. 



