142 PROCEEDINGS OF THE ACADEMY OF [1897. 



DEMONSTRATION OF ABSORPTION OF CARBON DIOXIDE AND OF 

 THE GENERATION OF OXYGEN BY DIATOMS. 



BY T. CHALKLEY PALMER. 



The essentially vegetable nature of diatoms is at the present time 

 acknowledged by biologists almost or quite without exception. The 

 phenomena of their increase and reproduction, if nothing else, are 

 of a nature to call for their grouping in the same class with such 

 undoubted plants as desmids and the Zygnemacea?. Yet every 

 young student, seeing for the first time the glassy cells of diatoms 

 moving about under his microscope in a manner that would seem to 

 indicate a very animal-like volition, is liable to ask for some tangible 

 proof of their plant nature, some more elementary argument than 

 that drawn from relations which are to be apprehended in all their 

 significance only after somewhat extended study. 



However difficult, or even impossible, it may be to draw a definite 

 line that shall separate the animal and vegetable kingdoms, it is 

 probable that no one will object if the term plant is applied to an 

 organism which, when exposed to sunlight, is found to absorb carbon 

 dioxide and to exhale oxygen. The method and apparatus de- 

 scribed herein are designed to show that both these phenomena, 

 which are so characteristic of plants in general, are characteristic of 

 diatoms. Pelletan 1 states that he has collected sufficient of the gas 

 arising from diatoms to serve for the application of those usual chem- 

 ical tests which prove it to be oxygen. But it is not easy to bring 

 together the conditions that permit the collection, from diatoms 

 alone, of such a volume of gas as is required for these tests. Con- 

 siderable time, also, must be needed for the operation. This question 

 of time is, in fact, important to the success of the experiment ; for 

 in the absence of sunshine, or at least of bright daylight, it is found 

 by experience that diatoms, and especially the motile forms that are 

 expending energy in the way made evident by their motion, cease 

 to exhale oxygen and begin to absorb it, or at least to give out 

 carbon dioxide. This phenomenon, — the evidence of an exothermic 

 chemical reaction, — the diatoms exhibit in common not only with 



Les Diatomes. 



