CHAPTER IV 



CYTOCHROME 



J. T is a fit subject for remark, if not for surprise, how few organic 

 substances figure prominently in both the animal and vegetable 

 kingdoms. One of the few is cytochrome, a substance which has only 

 recently been placed in its present important setting by the researches 

 of Keilin(i). Perhaps it is too much even to call it a substance, for 

 it may occur, as the seedsmen's catalogues say, "in variety," but 

 if we regard cytochrome merely as the name for a spectrum, that 

 spectrum is sufficiently widespread in its occurrence to justify the 

 name being placed at the head of the fourth chapter of this book. 



Let us turn to the spectrum and having gained familiarity with it 

 let us then discuss its significance. 



If the wing muscles of a bee be laid out on a glass slide in a thin 

 layer and exposed to air and looked at with a microspectroscope, no 

 particular spectrum will be seen. But if the muscles be covered over 

 with a slide, or otherwise protected from the oxygen of the air, 

 a band shortly appears ; in time this band becomes more intense, and 

 other bands, ultimately three of them, come into view till at last a 

 spectrum is seen which undergoes no further change. This spectrum 

 consists of four bands of different degrees of intensity, that seen first 

 being the most intense. It is the spectrum of cytochrome. 



Cytochrome is readily oxidised in the living cells, and in the 

 oxidised form it has but a poor spectrum. Therefore in the bee's 

 muscle exposed to air, the cytochrome being present in the oxidised 

 form, the absorption bands are hardly perceptible when such a pre- 

 paration is viewed through the microspectroscope. WheUj however, 

 a coverslip is placed on the muscle, especially if the latter be in fluid, 

 the muscular material reduces the cytochrome and, as the concentra- 

 tion of reduced cytochrome increases, the bands appear, the most 

 intense naturally being the first to make its appearance. 



That the number of bands really depends upon the quantity of 

 cytochrome in the layer of muscle traversed by the light on its way 

 through the instrument, may be demonstrated in a very simple 

 way. 



When the experiment which has already been described has been 

 performed (the cover glass being of sufficient strength), and all four 



