94 



CARNEGIE INSTITUTION OF WASHINGTON. 



atmospheric air of a soil-atmosphere rich in carbon dioxid. The fol- 

 lowing table summarizes two representative experiments with Mesem- 

 hryanthemum and illustrates the points: 



Table 1. — Groivth of roots of Mesembryanthetnum sp. in a soil atmosphere. 



In comparing the effects of the different percentages of carbon 

 dioxid on the three species, especially as regards the length of time 

 required in each case to bring about cessation of growth of the roots, 

 the results, although not altogether consistent in every instance, are 

 quite in harmony as a whole with the conclusions just presented. When 

 a gaseous mixture is used which contains 25 per cent carbon dioxid about 

 1.5 hours pass before growth of the roots entirely ceases in Covillea, 2.5 

 in Krameria, and over 4 hours in Mesemhryanthemum. In a 75 per 

 cent mixture, growth of the roots ceases at once in Covillea. In Kra- 

 meria, with the roots in a soil-atmosphere of this high carbon-dioxid 

 concentration at super-optimal soil temperatures, as above noted, the 

 growth of the roots may continue for 1.25 hours, but at lower, optimal, 

 temperatures of the soil the growth of the roots continues for a longer 

 time than this. In Mesemhryanthemum, however, an exposure of over 

 2 hours to a soil-atmosphere containing 75 per cent carbon dioxid is 

 apparently necessary, soil temperatures being most favorable for root- 

 growth, before growth of the roots ceases. 



In these experiments the amount of oxygen available for the roots 

 rapidly decreased with the increase in the amount of carbon dioxid 

 in the artificial soil-atmospheres used, so that to the specific effects of 

 carbon dioxid must be added the effects following partial deprivation 

 of oxygen, which, especially in the case of high percentages of carbon 

 dioxid, is an important factor. However, it is clear that in the case of 

 Mesernhryanihemum the roots are only slightly affected by the excess 

 of the one or by a deficit of the other. It is probable that aeration of 

 the roots of Mesemhryanthemum is not a factor of the first importance 

 in this species as it is in Covillea and Krameria. 



Size and Form of Leaves in Desert Plants, by W. A. Cannon. 



The well-known tendency of xerophytic perennials to have relatively 

 long leaves finds its ultimate expression in the vegetation of extremely 

 dry regions. A preliminary survey of the leaves of desertic and arid 



