66 THE SCOTTISH BOTANICAL REVIEW 



during decay therefore becomes a question of some import- 

 ance, since the pabulum necessary for organisms promoting 

 free-nitrogen-fixation would seem to be very limited. 



The vegetation of these dry limestone habitats is usually 

 stunted or dwarfed, doubtless due in part to drought, strong 

 illumination, or the effects of grazing by animals ; but it 

 seems to be a matter worth investigating whether this 

 dwarfing be due to these factors alone, or to further factors 

 amongst which poverty in available nitrogen may be of 

 importance. It has also to be shown to what extent the 

 xerophilous characters of these typically calciphile plants 

 may be a result of the concentration of mineral solutions in 

 the soil waters. 



If little loss of ammonia accompanies the rapid decay on 

 dry limestone soils, the latter may form a habitat for plants 

 peculiar in being fairly well supplied with ammonium salts, 

 but subject to a physical or physiological drought preventing 

 the rank growth usually found in habitats where there is a 

 liberal supply of nitrogen compounds. The peculiarities of 

 such plant societies would be, that they had plenty of avail- 

 able nitrogen, but avoided competition with the rank growth 

 found on all fertile soils with a good supply of water. If, 

 on the other hand, there is a considerable loss of ammonia 

 on plant decay, the habitat would be peculiar in being 

 subject to drought and nitrogen starvation, but retaining a 

 neutral or alkaline reaction of the soil. It is possible that 

 both conditions occur on dry limestone soils from differences 

 depending on such factors as the depth and nature of the 

 soil, and especially its degree of alkalinity, its clay and 

 humus content, and its local temperature or degree of 

 illumination, in their total effects on the retention of 

 ammonia and nitrification. 



Whether nitrogen-demanding or subject to nitrogen- 

 starvation, the plants of these dry limestone soils occupy a 

 habitat where the relations of calcium carbonate to fertility 

 are on an entirely different footing from those of the fertile, 

 moist, calcareous soils with abundant humus, and the so-called 

 calcicolous plants need critical analysis from this standpoint. 



A similar scarcity of humus occasionally occurs on bosses 

 of dolerite, where the soil covering the rock is very thin. 



