42 



supposition the first effect of the sunlight was the destruction of a 

 great number of the less resistant organisms, which accounts for the 

 slighter mass-development of the cultures after the 5 minute exposure. 

 For the remaining and more resistant cells we must then assume that 

 the actinic effect of a 15 minute exposure was stimulating, promo- 

 ting cell division, somewhat as exposure to increased osmotic pres- 

 sure 1 ) or to lack of oxygen, to heat, etc., induces it in unfertilized 

 parthogenetic eggs. As the eggs in these experiments must be 

 exposed only briefly and then returned to their normal environment 

 if maximal results are to be obtained, so with the bacterial cells. 

 The accelerating effect of the sunlight on growth does not seem to 

 be an enduring one, for later observations upon the same cultures 

 show that development is in reality permanently hindered; as much, 

 in the case of B. prodigiosus, B. ruber balticus, and B. 

 rutilus, by 5 minutes' exposure as by one of 30 minutes, and 

 nearly as much by 5 minutes as by one of 2 hours. 



The pigmentation of B. amyloruber and B. ruber balticus 

 also showed, in the 48 hour agar culture, a distinctly better color 

 after the 15 minute exposure. The chromogenesis of the others of 

 the series was markedly decreased by a 5 minute exposure, and only 

 B. ruber miquel showed any recovery after ten days. The greater 

 resistance of B. amyloruber, which was as brilliantly pigmented 

 after two hours' exposure to the sun as at first, was probably due to 

 its recent isolation from river water. 



No attempt was made to determine the further history of these 

 cultures. 



4. Summary. 



The experimental and comparative work done on these cul- 

 tures of pigment bacteria may be roughly summarized as follows: 



1) Notwithstanding the occasional loss of power of pigment 

 production by a previously chromogenic organism, the character of 

 the pigment is markedly constant among red chromogenic bacteria. 

 By constancy is here understood the appearance of pigment of definite 

 color upon nutritive media of known composition and under defined 

 environmental conditions. 



2) A collection of about forty red cultures selected at random 

 fell readily into four main groups, according to slight but constant 

 differences in the character of the pigment as determined by a 

 standard color scale. 



3) Sports, or discontinuous variations, such as white or light 

 colored colonies on a plate, viscosity of growth etc. sometimes oc- 

 cur. In general however the socalled important biological characters 

 are not subject to discontinuous variation. 



1) Loeb, J., Further Experiments on Artificial Parthenogenesis and the 

 Nature of the Process of Fertilization. (Amer. Jour, of Physiol. IV. 1900. p. 178.) 

 Matthews, A. P,, Some Ways of Causing Mitotic Division in Unfertilized 

 Arbacia Eggs. (Amer. Jour, of Physiol. IV. 1900. p. 343.) 



