Nov. 1S97.] BOTANICAL SOCIETY OF EDINBURGH 77 



curious circumstance that the bacteria, it" such is their 

 function, should exercise it through the thick, corky layer 

 of cells in which they are enclosed in the nodule, and that 

 as they increase in number, and correspondingly in their 

 demand for nitrogen, the wall surrounding them should be 

 gradually becoming less permeable. This is at variance 

 with what is found in other parts of the plant, such as in 

 the chlorophyll cells surrounding the stomata, where the 

 walls of the cells are made exceedingly thin, in proportion 

 to the activity with which gases are required to diffuse 

 through them. To discover whether nitrogen gas is 

 entering the nodule through the walls is a very difficult 

 matter. So far as it has been attempted by Kossowitch, 

 who grew nodulate plants in a soil supplied with an 

 artificial atmosphere composed of hydrogen and oxygen, but 

 containing no nitrogen, it has gone to show that nodules grow 

 independently of soil nitrogen. 



One would naturally expect that if the nodules were for 

 the purpose of absorbing nitrogen, they would be provided 

 with delicate hairs, or in some way present an easily 

 permeable membrane to the gas ; but there is no such 

 means provided, and there has been adduced no positive 

 evidence in favour of the view that the nodule is a gas- 

 absorbing structure. It has been suggested that the forked 

 bacteroids within the nodule arrange themselves in a loose 

 fashion, forming a network which presents a large surface 

 to nitrogen gas, after the manner of the lungs of animals. 

 This seems to me a fanciful notion, as it is highly improb- 

 able that a lung -like provision should be made for air 

 which was not allowed access thereto. 



AVe may now consider the other view held by Frank, 

 and in which he is supported by Schlosing and Laurent, 

 that the place where nitrogen assimilation takes place is 

 the same as that wherein carbon assimilation takes place, 

 viz. in the green leaves and other chlorophyll-containing 

 parts of the plant. In support of that view there is the 

 one very important piece of evidence, that the function 

 of absorbing and assimilating atmospheric nitrogen and 

 converting it into vegetable tissue has been shown by 

 Berthellot, and Andre, and others, to be possessed by 

 unicellular algie which inhabit ordinary soils abundantly. 



