2o8 HISTORY OF BOTANY 



in this branch of physiology between 1880 and 1885 was 

 devoted either to expositions of Pringsheim's theory or 

 to criticisms of his methods and refutations of his results, 

 but it would serve no useful purpose to follow the matter 

 further ; we may leave Pringsheim's theory and his 

 hypothetical first product of assimilation as, in Hansen's 

 words, an " unsubstantial dream." 



In 1880 A, J - W ^rbjrnper produced an elaborate 

 monograph on the structure and origin of starch grains 

 from chloroplasts, in which he showed that they might 

 also arise from what he termed " amyloplasts " in non- 

 green organs, and these, he affirmed, became chloroplasts 

 when exposed to light. In the following year Engelmann 

 drew attention to the behaviour of the motile form of 

 'Bacterium termo when placed in the vicinity of an illumin- 

 ated green cell. The movements of these organisms 

 were particularly active under red and orange rays, 

 very feeble under green, more vigorous under blue, but 

 ceased under ultra-red rays. The movements were 

 attributed to the extreme sensitiveness of the bacterium 

 to the presence of the most minute traces of oxygen, and 

 Engelmann suggested the employment of such motile 

 bacteria as tests for the evolution of oxygen, and con- 

 sequently of the intensity of photosynthesis in green 

 cells placed in different regions of the spectrum. This 

 method he elaborated in subsequent papers, and it has 

 been extensively used by many investigators since his 



day. 



Difficulties had been raised by several critics of 

 Baeyer's theory, on the grounds, well justified as it would 

 appear, that not only were carbon dioxide and water 

 very intractable substances and difficult to decompose, 

 but that carbon monoxide and free hydrogen could not 

 be detected, even in the faintest traces, in the fresh leaf. 

 Reinke met these objections, in 1881, by suggesting that 

 it was not carbon dioxide and water that were decomposed 

 but carbonic acid gas, which was a very unstable com- 



