BOT.-VOI,. 11] PEIRCE-FOOT-TUBERCLES. S'S 



heavy, compact, clay soil. It also varies with the tillage 

 I have found tubercles much lower on the roots of Bur 

 Clover growing on a heap of gravel than in an undisturbed 

 and compact day. Mechanical reasons are msuffioen to 

 account for this. The only inference to be drawn is that 

 the bacteria are limited in their natural distribution to those 

 soils and those layers of soil which contam considerable 

 volumes of air, for only there will they find enough oxygen 

 and nitrogen for their needs. . 



It is the general habit of leguminous plants to send their 

 roots fairly deep into the soil. In a natural field, or one 

 returned to a state of nature, where the soil is covered by 

 a mixed vegetation, it is found that different plants send 

 their roots to different depths. In this way the resources 

 of the soil are more perfectly exploited by the plants and 

 destructive competition is avoided. But it is to be noted 

 that of those plants which send their roots deeper, many are 

 members of the Leguminos,^. Is this merely to escape 

 coffipetition with other seed-bearing plants, or to reach a 

 more abundant and constant supply of water, or to escape 

 the attacks of the bacteria which cause them to form tuber- 

 cles' It is mainly the Leguminosa, which are successfully 

 attacked by tubercle bacteria, and they as a rule, send 

 their roots fairly deep imo the soil. Furthermore, the 

 number of roots increases with the distance from the sur- 

 face It would appear not inconsistent with the evidence 

 so far obtained, to suppose that the habit of the Legumtnos. 

 of sending their roots deep, and of causing them to branch 

 copiously only after they have reached some depth is one 

 means which these plants have of avoiding the attack of 



tubercle bacteria. 



Alfalfa or Lucern ( Medlcago sativa Linn.) has notori- 

 ously long roots. They are reported to go down to the 

 permanently water-bearing levels of the sod. The best 

 chance for examining these roots would be offered when a 

 well is being dug where Alfalfa is growing. I have not 

 been so fortunate as to have such a chance, but digging 

 around Alfalfa plants growing as weeds in a grass field 



