42 THE BIOLOGY OF FLOWERING PLANTS 



roots to grow rapidly into the moister layers of the soil 

 below the surface." 



4. The hydrogen ion concentration of the soil has, 

 undoubtedly, an important effect on root development. 

 This is clearly shown by experiments carried out by Olsen 

 (1923) on the growth of various plants in culture solutions 

 identical except for their p^ values. Table VIII shows 

 the dry weight attained by Senecio sylvaticus, which occurs 

 naturally in acid soils of ^" 4*9-5 '6, and by Tiissilago 

 Farfara, which grows in alkaline soils of ^^ 7"o-7*9- 



TABLE VIII 

 Hydrogen Ion Concentration and Root Development 



p^ of nutrient solution 



Dry weight in 1 

 grammes of I Senecio 

 root sys- jTussilago 

 tern I 



3"o I 3"S 



0*3 2'2 



— I o*i 



4-0 



3-4 



0-8 



4*5 



I'2 



S'o 6-0 



2"3 i 2"0 



1-4 ! 2-5 



6-5 



7-0 



0"2 



2'5 



7'S 



This shows the effect of the p^ value on a particular species 

 and also the different reaction of species naturally occurring 

 in soils of different acidity. It does not, of course, enable 

 us to say how far the effect is due to direct action on the 

 roots. 



Tottingham and Rankin (1922) find that growth in 

 length of the roots of wheat is maximal in solutions of p^ 

 7-5. At that value the roots in their cultures attained 

 a length of 30 cm. as compared with 24*5 cm. at p^ 6*4 

 and 10-6 cm. at p^ 5*3. The greatest dry weight, how- 

 ever, was made at p^ 6-4. Hoagland (1917) found the • 

 roots of barley to be injured at /)" 7'4, to show good 

 growth from ^" 6-i6 to p^ 7*07, with a maximum at 

 the former, and injury again at p^ 3 •54. These results 

 are as yet not very extensive, but the immediate effect 

 of the p^ value of the soil on that of the cell sap makes 

 it certain that such reactions as growth rate must be affected. 

 The hydrogen ion concentration of the soil is evidently 



