394 Scientific Proceedings, Uoyal Dublin Society. 



rule the records relate to common and widely spi-ead plants, for it is reason- 

 able to suppose that the absence of these is in any region due to some very 

 definite factor, whereas with the rarer plants it is rather the concurrence of 

 a number of favouring circumstances that renders growth possible. A few 

 of the latter have, however, been mentioned. 



With regard to the limitation of the distribution of the weeds of arable 

 land to particular types of soil, a perusal of the lists given by Brenehley 

 (1911, '12, '13) leaves one with the impression that it is hard to draw definite 

 conclusions on the subject, the same plant being met with in very varying 

 soils. Thus, a typical plant of limestone soil may be found in sandy soil, and 

 on examination it has happened that the two soils had closely similar pH 

 values, so far as the writer's experience has gone as yet. 



When considering the relation of plant distribution to the hydrogen ion 

 concentration of the soil solution, it is well to avoid the chastisement with 

 scorpions of the modern ecologist by recalling a paper by Foot (1865) : " All 

 old floras dogmatically tell us Digitalis avoids the limestone. It is, doubtless, 

 more plentiful on sandstone and clay formation, bi^t grows on limestone also. 

 It is abundant about MuUingar, also occasionally in Burren." The limits 

 given for a species are accordingly only to be taken as embodying the results 

 obtained so far. They do not mean that the species is never found in nature 

 outside these limits : still less do they mean that it cannot grow in water 

 culture outside the limits given. Growth in soil is complicated by many 

 factors. Thus, Hartwell and Pember (1918) have shown that acid soil affects 

 barley and rye differently, and that this is due to tlie aluminium sails, which 

 do not harm the rye, but injure the barley. The addition of acid phosphate 

 made the soil more acid, but the barley then grew well, as aluminium phos- 

 phate was precipitated. Miyake (1916), too, showed how toxic minute traces 

 of aluminium salts were to rice. The effect of the acid is clearly an indirect 

 one in these as in many other cases. This has recently been emphasized by 

 Mirasol (1921), and Comber (1921) instances the different behaviour of a 

 clay soil from that of a peaty soil of equal hydrogen ion concentration. But 

 bearing in mind such very necessary limitations to its application, it is still 

 true that the hydrogen ion concentration may be taken as an index of a set 

 of conditions, physical and chemical, which control plant distribution. Thus, 

 in a soil rich in calcium, the pH value is an index of soil aeration. When 

 this is good the soil will give a reaction close to that for calcium bicarbonate 

 in equilibrium with the air, pH 8'3-8'4, but as aeration becomes less and 

 carbon dioxide accumulates in the soil atmosphere to the extent of 0'25 to 

 one per cent, or over, the pH value decreases. Where there is only a small 

 amount of bicarbonate the soil may even become acid from this cause. The 



