RED PIGMENT OF FLOWERING PLANTS. 411 



medium for the toxic rays. It may be confidently stated 

 that ordinary anthocyan obtained from various plants would 

 be by no means serviceable in this respect, as the curves of 

 anthocyan absorption figured by Engelmann show, and as 

 will be readily understood when the absorption spectrum of 

 anthocyan is later described. It is true that violet cell-sap 

 absorbs more of the violet end of the spectrum than does 

 red-coloured anthocyan, but even this colouring matter 

 intercepts still more of the red end of the spectrum than of 

 that rich in toxic rays. 



The screen theory must then, for the present, be regarded 

 as non-proven. Stahl has criticised Kerner's view very 

 adversely. He inclines to the belief that too much weight 

 has been attributed to the destructive effect of light on 

 chlorophyll ; whereas the thermal effects, helpful or harmful, 

 have not received sufficient attention. He adds the further 

 criticism that, before Kerner's conclusions from his Alpine 

 experiments can be accepted, other researches must be made, 

 some at still higher altitudes, others in which some of the 

 plants are protected at night ; for as will be seen when 

 Stahl's own work is considered, the death of the Linum 

 plants may be due to a cause other than that of too intense 

 light. The consideration of the other two of Kerner's con- 

 jectures concerning the functions of anthocyan may be 

 postponed till the researches, in which they are respectively 

 re-stated are discussed. 



The absorption spectrum of the red-colouring matter of 

 leaves has been very fully investigated by Engelmann (25). 

 He finds a general agreement in this respect between the 

 red pigments of different plants. The absorption by these 

 red pigments is greatest in the green but extends as far as 

 the D line on the one side and beyond F, although here 

 very feeble, on the other ; beyond G again there is some 

 little absorption. Thus the absorption spectrum of antho- 

 cyan is practically the reciprocal of that of chlorophyll. 

 Engelmann remarks that in many plants the chlorophyll 

 grains live permanently behind red screens of such ab- 

 sorbent power that ^ to i of the total light is absorbed, 

 without assimilation being in the least impaired. 



2<S 



