EEPRODtTCTlON OF WESTERN YElXOW PIITE. B*7 



rock surfaces and even penetrate the pores. Veatch (25) cites an 

 instance in 1919, when after a prolonged rainy period, the stony clay 

 soils near the Fort Valley Experiment Station were penetrated by 

 moisture to a depth of 3 feet or more, whereas a stone-free clay was 

 penetrated only from 12 to 13 inches. This condition, however, ap- 

 plies only during the period of siunmer showers. The fine clays, as 

 well as other soils, are usually saturated to a great depth by winter 

 precipitation. 



Germination also is closely related to the texture of the soil. Fine 

 clay soils usually form a smooth, compact surface, especially if 

 tramped by grazing animals. Seeds are not readily covered, and if 

 they are covered the clay forms a hard crust which the tender seed- 

 ling may not be able to break through. These adverse conditions 

 are relieved by gravel and stones mixed in the surface layer or 

 loose on the surface. This subject is treated more fully under " Her- 

 baceous vegetation." 



ROOT DE^^LOPMENT. 



The survival of young seedlings during critical droughts is deter- 

 mined by the amount of available moisture within the soil mass occu- 

 pied by their roots. Obviously a seedling whose roots are not in 

 contact with a fairly constant source of moisture has a poor chance 

 for survival. Not only depth of roots but also lateral extension is 

 important, because, other things being equal, the amount of mois- 

 ture available is proportional to the volume of soil drawn upon. 

 In view of these facts, a study of the form and depth of the root sys- 

 tems of pine seedlings in various soils may throw considerable light 

 upon the reproduction problem. 



Haasis in 1919 and 1920 excavated the root systems of some 200 

 western yellow pine seedlings, ranging from 1 to 5 years of age. He 

 found the shallowest penetration in the gravelly and loamy soils, 

 the deepest in fine clay, and intermediate penetration in stony clay. 

 Lateral development was least in the fine clay, greatest in gravelly 

 soils or cinders, and intermediate in stony clay. He formulated the 

 general rule that the more compact the soil the deeper and less 

 branching the root, and the looser the soil the shallower and more 

 branching the root. This is contrary to the observation that roots 

 of mature trees do not penetrate deeply in clay soils because of the 

 compact structure. In the case of trees, however, w^e are dealing 

 with greater depths, down to 3 or 4 feet. It is possible that the 

 failure of roots to penetrate below this depth may be due to other 

 causes such as insufficient aeration. Moreover, bedrock or hardpans 

 are usually encountered at less than 4 feet. In the light of Haasis's 

 investigations it appears that the roots must penetrate deeply in 

 clay soils in order to secure moisture, and that the seedlings which do 



