TISSUES AND SIMPLE ORGANS. 129 



elements (7, //, 6>, and N, iron is necessary to its development (as 

 well as to its composition?). The plasmatic colorless or nearly 

 colorless basal substance (stroma) of the chlorophyll-body is tinged 

 with the green coloring substance ; this latter can be extracted with 

 alcohol. The delicate structure of this fundamental substance 

 according to more recent authors is said to be spongy, not homoge- 

 neous. The fact that chlorophyll-bodies divide has been known 

 for some time. Further, it has been supposed by many authors 

 that two coloring substances, a green and a yellow, are present in 

 the chlorophyll-bodies (according to earlier investigators, blue and 

 yellow). The foregoing statements represent, so to speak, succes- 

 sive stages, which are not yet concluded, of the attempts made to 

 find the. chemical and physical structure of chlorophyll-bodies. It 

 is to be kept in mind at present that chlorophyll is a green-colored 

 plasm of highly characteristic properties which manifest themselves 

 in the work of assimilation. 



In regard to this work of assimilation we must, in view of 

 the results obtained by ENGELMANN (Utrecht), admit that con- 

 siderable progress has been made. The theory of the physicist 

 LOMMEL that the rays which are absorbed by the chlorophyll-spec- 

 trum are most active in assimilation seems to have been verified by 

 Engelmann. The method of investigation of this latter physiolo- 

 gist is in itself very interesting. It is called the ' ' bacteria method, ' ' 

 and consists in its essentials of the utilization of sensitive bacteria 

 suspended in a drop of water. The bacteria accumulate where 

 there is a supply of oxygen. An assimilating cell-thread under 

 the microscope is observed under such environments as expose it 

 to the seven colors of the solar spectrum which are projected side 

 by side on the long axis of the thread ; the surrounding liquid con- 

 tains the sensitive bacteria ; they accumulate most at the points of 

 maximum assimilation, hence where the most oxygen is liberated. 

 These experiments show that the two optima of assimilation (as 

 judged by the liberation of oxygen) occur first in the red and a 

 second smaller optimum occurs in the highly refrangible parts of 

 the spectrum : blue, violet, and ultra-violet ; it is in these spectral 

 areas that the characteristic absorption-bands of chlorophyll lie 

 (similar to those of living chlorophyll). (The optimum of assimila- 

 tion in the red [orange] had been observed by BEINKE, previous to 

 the investigations of Engelmann, and still earlier by ~N. J. C. 



