98 THE POPULAR SCIENCE MONTHLY 



We can only consider briefly the root-air relation, since little work 

 has been done on the soil atmosphere. We therefore do not know the 

 rate of movement of air in the soil, or, for that matter, its composition. 

 It is probable that there is a large per cent, of carbon dioxide where 

 there are a relatively large number of roots of plants, bat as to the 

 diffusion of oxygen from the air into the soil or the diffusion of carbon 

 dioxide from the soil into the air, little appears to be known. Prelim- 

 inary tests show that there may be more movement of the air in the 

 soil than might at first be supposed, and that varying, even if small, 

 atmospheric pressure may directly affect air movements in the soil. 

 For examples, if a tube 50 cm. long and 2 cm. in diameter be filled with 

 soil composed of sifted sand and adobe — one part of the former to two 

 parts of the latter — it will be found that a water pressure of only 1 cm., 

 or less, will be required to force a continuous stream of air through it. 

 The pressure given is for soil saturated with water. When air-dry, 

 there is almost no resistance. In soils of this composition, therefore, 

 it is probable that ordinary variation in atmospheric pressure is suffi- 

 cent to induce in it rapid air movements. Preliminary experiments, in 

 which a stream consisting of 20 c.c. of air a minute was passed through 

 the soil where the roots were placed, indicated by the great vigor of the 

 plant, and the relatively extensive root development, that that amount 

 of air was beneficial to development and forwarded growth. Variations 

 in temperature with depth of soil, variations in water content of the 

 soil, are both additional potent factors in modifying the rate of move- 

 ment of the soil air. 



While it is not known in an exact way how the atmosphere of the 

 soil effects the position or certain other features of the root-systems of 

 plants, it seems probable that in certain cases, at least, the effect is 

 pronounced. For example, as has been shown above, the root-systems 

 of the cacti without exception are placed near the surface of the ground. 

 The roots grow in a soil horizon which is not the most moist, but, on the 

 other hand, which although moistened first is also the first to give up 

 its water, and it very likely is the optimum air content of the soil at a 

 critical period which determines the superficial placing of the roots. 

 It is a well-known fact that many bulbous plants require well-drained 

 soil, which is probably only another way of saying that they thrive best 

 in soils having good aeration. Two or three experiments may be cited 

 which may be taken to substantiate the conclusions just stated. For 

 example, there grows in the vicinity of Tucson a cylindro-opuntia 

 (Opuntia arbuscula?) in which the root-system is fleshy, the roots 

 having much the appearance of slender sweet potatoes. It was supposed 

 at first that the fleshy roots of these species was a specific character, 

 which, indeed, may be true, and therefore obligate. Some doubts, how- 

 ever, have been thrown on this conclusion from observations on another 



