FACTORS IXFLUEXCINO REPRODUCTION 



847 



two sets of measurements. In the top measurements (figs, i and 2) 

 the mineral soil showed a little more rapid growth than the raw humus, 

 while in the root measurements the mineral soil falls far behind the raw 

 humus. The most probable explanation is that the plants on the min- 

 eral soil were injured by a hot, dry period of 16 days in the middle of 

 August. The soil of the flats, being only 8 centimeters deep and cut 

 off from capillary water, was subject to rapid drying out in spite of the 



Date of Measurement 

 Fig. 3. — Influence of soil on the growth of corn. See figure i for acidities. 



covering of half shade. Under these conditions the mineral soil, hav- 

 ing the lowest moisture-holding capacity, dried out more rapidly than 

 the other soils. Another factor which may also partly explain the in- 

 ferior root development in the mineral soil is the poorer aeration of 

 this soil. Since the mineral soil, though decidedly less aerated than the 

 mild humus and raw humus, was not a heavy soil, we have another 

 indication of the need for ample soil oxygen on the part of conifers.^^ 



" Boerker, R. H. : "Ecological investigations upon germination and early 

 growth of forest trees." Univ. of Nebraska, Jan., 1916. 



