DEPARTMENT OF BOTANICAL RESEARCH. 93 



of growth and the character of the roots are concerned, which have been 

 pubhshed in various studies, it need only be said here that Covillea is a 

 sclerophyllous shrub, without water-storage capacity, which occurs in 

 relatively shallow and fairly hght soils, with relatively shallowly placed 

 roots, but which also grows along water-courses where the soil is 

 deep and sandy. Under such conditions the roots may penetrate to a 

 depth of 2 meters, or possibly more. Krameria is a root parasite, with 

 Covillea, among other perennials, as one of its hosts. It bears leaves 

 which are deciduous and the root system is relatively shallow, appar- 

 ently under any conditions. It is also without water-storage capacity. 

 Mesenibryanthemum is a succulent with shallow roots, generally grow- 

 ing in sandy soil. 



In the experiments on the root-carbon-dioxid relations the roots 

 of the three species were treated in a similar manner. They were 

 grown in glass tubes which were filled with fairly coarse sand and which 

 were kept in thermostats in which the temperature of the soil varied 

 scarcely if any more than 2° C. The shoots projected into the air and 

 were subject to daily fluctuation in temperature amounting to about 

 5° C. Artificial atmospheres were employed which consisted of 25, 

 50, and 75 per cent carbon dioxid, the balance being atmxospheric air. 

 This was used to replace the normal soil-atmosphere, and at the end 

 of the experiments it in turn was replaced by atmospheric air. The 

 leading results of the experiments can be briefly given. In the case 

 of Krameria, carbon dioxid in any percentage exercised a depressing 

 effect on the rate of growth of the roots, and, in fact, growth of the 

 roots ceased after a time, which was longer or shorter in accordance 

 with the duration of the exposure to the gaseous mixture as well as 

 with the percentage of carbon dioxid used, but after the admission of 

 atmospheric air, the growth of the roots was resumed. In a soil- 

 atmosphere containing 75 per cent carbon dioxid it was observed in 

 Krameria that the effects were less harmful at soil temperatures which 

 were approximately optimal than it was at temperatures of the soil 

 which were about 10° C. above this, or in other words, that about 

 optimal for root-growth of its hosts. In Covillea, carbon dioxid of 

 whatever concentration used was disastrously harmful. The growth 

 of the roots was quickly retarded and soon ceased. In case of recovery 

 upon the admission of atmospheric air it was seen to occur many hours 

 subsequent to the employment of the artificial soil-atmosphere con- 

 taining the high percentages of carbon dioxid. As to Mesembryan- 

 themum, the effects following the use of soil-atmospheres containing 

 large amounts of carbon dioxid, in certain respects, were different from 

 the effects observed in the other species treated. In Mesembryanthe- 

 mum the rate of growth of the roots is decreased in atmospheres con- 

 taining 25 to 75 per cent carbon dioxid, and finally, after relatively 

 long exposures to the higher concentrations, it ceases. However, it 

 was seen to be renewed relatively soon upon the replacement by 



