118 



and alcohol. In opposition to this view, Beijerinck maintained that 

 zymase is an essential, microscopically visible part of the yeast proto- 

 plasm, and therefore occurs in the maceration juice as a suspensoid. 

 Although later investigations have more or less justified this opinion, 

 the conclusion of the authors that zymase will never pass undamaged 

 cell walls needs further confirmation. 



Beijerinck's last contribution i) to our knowledge of the yeasts 

 dealt with a noteworthy phenomenon, the cause of which is not yet 

 fully understood. Many yeast species are known which owe their red 

 colour to the presence of a pigment of carotenoid nature. Beijerinck 

 now made the observation that several yeast species — as, forinstance, 

 Saccharomyces pulcherrimus and various yeasts isolated from milk — 

 which under normal conditions are colourless, produce a red pigment 

 only when grown on media containing somewhat larger quantities of 

 iron salts. The nature of this red pigment is as yet unknown, but in 

 any case it is not related to the carotenoids. 



f. Beijerinck's contribution to the virus concept. 



In 1898 Beijerinck published a paper 2) which has since made him 

 known as one of the pioneers in the field of virus study, so important 

 nowadays. The paper deals with Beijerinck's observations on the 

 tobacco mosaic disease. In it ample proof is afforded that the conta- 

 gious agent causing the disease does not belong to the visible micro- 

 organisms, but on the contrary is a principle which occurs in the plant 

 juice in a "dissolved state", i.e., passes filters which retain all mi- 

 croscopically visible particles. 



In the introduction to the paper Beijerinck states the reasons 

 which led him to his investigation. They seem sufficiently interesting 

 to report them briefly here. In 1885 while he was still working in the 

 Agricultural College at Wageningen, his colleague Adolf Mayer 

 brought experimental proof for the contagious character of the mosaic 

 disease. At Mayer's request, Beijerinck made an attempt to isolate 

 the responsible micro-organism, but the result of his investigation was 

 entirely negative. However, on account of the very restricted bacter- 

 iological experience which he possessed at that time, Beijerinck 

 himself did not consider this result to be conclusive. The successful 

 isolation of the root nodule organism in 1 887 encouraged him to make 

 another attempt to isolate the causative organism of mosaic disease 

 in tobacco. The consideration that it was not the special task of an 

 industrial microbiologist to solve the riddle of a contagious plant 

 disease does not have seem weighed heavily with Beijerinck. This 



1) Arch. néerl. d. physiol. de rhomme et des animaux 2, 609, 1918. 



2) A preliminary publication appeared in Versl. Kon. Akad. v. Wet. Amsterdam 7, 

 229, 1898. A more extensive memoir was published in Arch. néerl. d. sciences exactes 

 et naturelles Sér. II, 3, 164, 1900. 



