78 CARNEGIE INSTITUTION OF WASHINGTON. 



penetrated to a depth of about 80 cm., and in the following August a 

 depth of 1 meter was attained. The shoots were approximately 6.5 

 cm. in length a year after the seed was sown. 



The results observed in the outdoor cultures, therefore, confirm the 

 results of laboratory experimentation. Numerous tests have indicated 

 that the roots of cacti from the Tucson region and from the Mohave 

 grow comparatively slowly at soil temperatures below 20° C. The 

 factor limiting root-penetration in these forms at the Coastal Labora- 

 tory, consequently, is the relatively low soil-temperature that obtains 

 at that place. On the other hand, it has been seen in many experiments 

 that the roots of Prosopis have an effective rate of growth at tempera- 

 tures under 20° C, although the optimum, as in the case of the cacti, 

 is much above this. The soil-temperature at depths thus far studied 

 is well above the minimum for root-growth of Prosopis. We find, 

 accordingly, that the roots of this species penetrate the soil at Carmel 

 relatively deeply. 



From the observed behavior of the roots of Pistacia at Carmel it is 

 concluded that the temperature reponse of the roots of the species is 

 similar to that of the roots of Prosopis. Thus, it is because the roots 

 of Pistacia have a fairly active growth-rate at relatively low soil- 

 temperatures that in their proper habitat they penetrate to the soil 

 horizon carrying moisture throughout the year. The characteristic 

 temperature reaction of the roots of Pistacia, therefore, constitutes a 

 vitally important adjustment of the species to its difficult environment. 



Relation of Sail Aeration to Plant-Growth, by B. E. Livingston and E. E. Free. 



In this report for 1915 announcement was made of experiments on 

 the amount of oxygen necessary for the roots of Coleus growing in soil.^ 

 These experiments have been continued with an improved and simpli- 

 fied technique and tests have been made also of heliotrope {Heliotro- 

 pium peruvianum), oleander {Nerium oleander), and swamp willow 

 (Salix sp.). Heliotrope is found to resemble Coleus in requiring the 

 maintenance of a certain oxygen-content in the air of the soil. The 

 roots of oleander also require oxygen, but appear to need much less of 

 it than do the roots of Coleus and heliotrope. Also the injury produced 

 by entire deprivation from oxygen is much less quickly developed. The 

 roots of the swamp willow appear to be unaffected by oxygen depriva- 

 tion. Plants grown in soil the atmosphere of which contained no 

 oxygen showed normal behavior and growth over periods as long as 10 

 weeks. It appears that plants of different species may differ widely as 

 to the oxygen required by their roots. Together with the work of 

 Cannon, reported in preceding paragraphs of this report, this indi- 

 cates that soil aeration is probably a much more important ecological 

 factor than has been suspected. 



'Carnegie Inst. Wash. V<\'ir Rook for 191.'"). pp. fi()-f)l. 



