102 Otis F. Curtis 



much more slimy, with a mat of growth around the stem; yet the roots 

 of the latter, were much better developed. When measured later, on 

 March 11, the twigs treated with permanganate and sugar had a root 

 growth 3.39 times that of the twigs treated with sugar alone (fig. 6), 

 which were noticeably less sUmy; and the former had a growth 1.94 times 

 that of the plants not supplied with sugar, the stems of which were not 

 sUmy to the touch. It would seem, then, that the growth of microorgan- 

 isms does not of itself injure the cuttings, but that for some reason they 

 become weakened and die, following which fungous growth sets in, 

 attacking the dead twigs. 



It, is very probable that the presence of soluble organic material allows 

 for a much increased growth of microorganisms,- which would result in 

 an increase of carbon dioxide and various toxic compounds and a decrease 

 of oxygen. This increase of carbon dioxide has been found to occur in 

 soils wher^ green crops are plowed under or in ground that has been 

 heavily manured (Boussingault and Lewy, 1852, and Lau, 1906). Kidd 

 (1914) found that the carbon-dioxide content of the soil was very much 

 increased when clipped grass was buried in a pit beneath it; after seven 

 months the soil air of such a pit contained 8 per cent of carbon dioxide 

 As stated earlier, De Saussure (1804) found that an increased carbon- 

 dioxide content retarded root growth in seedlings. Boehm (1874), Chapin 

 (1902), and Cannon and Free (1917) also have reported retardation of 

 root growth in the presence of carbon dioxide. 



It would seem very probable that the increased carbon-dioxide content 

 resulting from the action of microorganisms on the organic material in 

 the medium is injurious to cuttings. Kidd (1914) has shown that carbon 

 dioxide has a retarding effect on respiration, and that this effect is enhanced 

 by a decrease in oxygen content. Microorganisms growing in a medium 

 containing organic matter not only increase the carbon-dioxide content 

 but also decrease the percentage of oxygen. This decreased oxygen 

 supply, coupled with an increased carbon-dioxide content, would tend 

 to retard respiration or to retard the further oxidation of toxic products, 

 thereby causing the death of the twigs, which later might be attacked 

 by saprophytic forms or might be so altered as to be easily attacked by 

 semiparasites. 



Furthermore, Chapin (1902) observed that the roots of plants are 

 more resistant to a high carbon-dioxide content than are the tops. This 



