68 Richard H. Boerkcr 



In connection with the soil moisture experiments a very in- 

 teresting fact was noted. Both Pinus ponderosa and Robinia 

 pseudacacia wilted on January i, just exactly two months after 

 the seeds were sown. The soil moisture at the time was de- 

 termined to be 6.6 per cent. It happens that at three different 

 times the moisture content was far below this figure. On Oc- 

 tober 28 the seeds were sown, on November 7 the moisture 

 content was 4 per cent., on the nth it was 4.6 per cent, and again 

 on December 5 it fell to 6.1 per cent. Robinia pseudacacia ger- 

 minated first on November 9 and the Pinus ponderosa on No- 

 vember 26. It is evident from this occurrence that more mois- 

 ture is needed for the early development of the seedlings than is 

 necessary for germination. On the 9th of January this fact was 

 further emphasized. While taking root and stem measurements 

 and digging up the seedlings two germinating seeds of Robinia 

 were found. The moisture samples taken on this day show 5.7 

 per cent, moisture in the dry culture. As a result of this condi- 

 tion no stem and root measurements appear in the dry column at 

 the age of three months. 



In all species measured the length of the stem decreases with 

 diminishing moisture supply and the fact is noted that this de- 

 crease is greater in the case of Robinia than it is in the case of 

 Pinus ponderosa or Pinus strobus. This indicates the greater 

 drought resistance of the conifers as compared to the hardwoods. 



In the case of the root development of Pinus ponderosa it is 

 shown that both the tap root and the total length of lateral roots 

 increase with diminishing moisture supply. For Robinia the 

 result was quite different, for it was found that the length of the 

 tap root decreases with diminishing moisture. While Pinus 

 ponderosa seems to be able to develop roots to reach the lower 

 moisture layers of soil, Robinia is unable to do this. 



The following table gives the results on the effect of soil tex- 

 ture upon the development of the stem and roots of these species. 



The greatest length of stem in Pinus ponderosa was found to 

 be in the case of the two-months-old seedlings in the loam and the 

 next greatest length in the gravel. In the case of the three- 

 months-old trees the greatest length was in the gravel and the 



