30 



You now have such a flask with an elongated neck as is fig- 

 ured in the text-books, but with the advantage that, the neck be- 

 ing in two pieces, the potash can be introduced in the manner 

 described with the greatest ease. As the carbon dioxide is 

 evolved, it, of course, sinks and is absorbed by the alkali, the 

 diminution in volume of the air within the flask being shown by 

 the corresponding rise of the mercury in the neck. The tube be- 

 ing graduated, this absorption can be readily observed and noted. 



The apparatus is so simple and so quickly and cheaply put to- 

 gether, that several may be set up side by side for comparison of 

 the respiration of different plant organs under different condi- 

 tions. There are certain errors to be guarded against. The rise 

 of the column within the neck should be read from the mercury 

 surface and not from that of the potash above it, since the latter 

 absorbs water as well as carbon dioxide. Great change of tem- 

 perature should also be guarded against, since it alters the vol- 

 ume of gas within the flask, and to this the apparatus is very 

 sensitive. If the volume of the flask and neck, less the contents, 

 be taken, the proper correction for temperature variation may be 

 applied. There is also an error introduced by the variation of 

 vapor tension due to the possible change of the moisture in the 

 enclosed air, but this is not considerable. Such a union by a 

 ground joint is vastly to be preferred to cutting off the flaring 

 mouth of the flask and uniting it to a tube by a rubber joint. 

 The manipulation of a rubber tube of such size and under such 

 conditions is annoying in the extreme. The ground joint pre- 

 pared as above and well sealed with vaseline has proved itself to 

 be entirely air-tight. 



THRIVING UNDER DIFFICULTIES 



By David Griffiths 



A sewer pipe three feet underground is not only uninviting but 

 rather an unusual object of botanical study. Yet associated with 

 such a structure on the campus of the University of Arizona, the 

 workmen uncovered one of the most interesting struggles for ex- 



