472 BROOKLYN BOTANIC GARDEN MEMOIRS 



The normal or check series were run in water alone. The results 

 given by these experiments, which were limited in numbers, are 

 similar to those obtained by the use of excelsior in contact with roots 

 in water, namely, the various grades of gravel, sand, and silt gave 

 rise to different reactions on the part of the organism which resulted 

 in a stimulated growth correlation, as exemplified in the development 

 of the primary organs and secondary root system. The results ob- 

 tained by the use of mustard, peas and soy beans, etc., in the experi- 

 ments so far carried on, indicate generally that there exists little 

 difference in the height of the hypocotyls or stems of the normals, 

 and those in contact with the dififerent-sized particles of soil constit- 

 uents. The growth in length of the primary root is lessened and the 

 secondary root system is greatly increased as we approach the finer 

 grades of contact material. In some species with relatively large 

 roots the coarser particles, namely 16-8 mm., appear occasionally to 

 stimulate primary organs, but this reaction is variable, and associated 

 with growth correlations as manifested in the more or less increased 

 development of the secondary root system. The reaction of roots 

 to different-sized particles will undoubtedly be found quite variable. 

 Species with relatively large and strong primary roots such as the horse 

 bean, peas, etc., react differently from seedlings with a delicate root 

 system, such as the mustard and turnip. The roots of the latter 

 species, even when grown in contact with excelsior or soil particles, 

 produce a marked secondary root system characterized by little or 



Table 19 



Showing Growth of Hypocotyls and Roots of Mustard {Brassica alba Boiss.) in Contact 



with Different-sized Particles of Sand and Gravel in Wattr^^ 



Average Length of Hypocotyls and 

 Primary Roots (Centimeters) 

 Size of Particles 

 (Millimeters) Hypocotyls Roots 



" 7 i2.7 



2 12.6 



7 8.7 



Normal 



16-8 8 



8-4 II 



4-2 II 



2-1 II 



1-0.5 II 



0.5-0.25 II 



0.25-0.1 13 



5 7-6 

 9 44 



6 4-3 

 2 4.2 



7 3-4 



no primary root development. The same tendency exists in species 

 characterized by larger roots to increase their secondary root system. 

 On the other hand, the primary root system is greatly accelerated by 

 contact with excelsior in species with large roots. (Cf. Tables 16-17 

 and 18.) The experiments with mustard given in Table 19 show 



" 15 plants used in each test. 



