THE FACTORS OF DISTRIBUTION 53 



to the plant, but it appears that difficulties in cultivating 

 certain orchids and other plants are very probably due to 

 the absence of the suitable mycorhizae. They seem to 

 assist the plant to utilise carbonaceous matter in the 

 humus, and perhaps also at least in some cases to fix 

 atmospheric nitrogen. Pinus montana Duroi has both 

 mycorhizae and root- tubercles ; and not only is it said 

 to fix free nitrogen, but also to benefit the growth of 

 the Spruce when, as in Jutland, the two species are 

 associated. 



RESPONSE OF VEGETATION TO SOIL. The general 

 response of the vegetation to soil-conditions forms the 

 main subject of ecology, and is dealt with later; but we 

 must here notice some cases where it can apparently be 

 definitely assigned to some of the factors that we have 

 just been considering. The incoherent, non-retentive 

 character of sand is reflected in the deep roots, long 

 branched rhizomes, and inrolled leaves of the dune 

 grasses and sedges, such as Ammophila arenaria Link, 

 Elymus arenarius L., and Car ex arenaria L., typical 

 xerophytes, though specially separated as psammophytes 

 or sand-plants, or psammophilous (sand-loving) plants. 

 Some species common on sands are so because of their 

 calciphobe or calcifuge (lime-fearing or lime-avoiding) 

 character. Among these are the Spanish Chestnut 

 (Castanea sativa Mill.), the Cluster Pine (Pinus mari- 

 tima Lam.), the Foxglove (Digitalis purpurea L.), the 

 Broom (Sarothamnus scoparius Wim.), Gorses, Heaths, 

 Sundews, and Bracken. Though all these species may 

 be termed silicicolotis , or silica-inhabiting, considering 

 the insoluble and physiologically useless character of 

 silica, it must be the physical character of sandy soil, 

 rather than any chemical constituent that can act as 

 a positive attraction. 



Lime, though in these last-mentioned instances a 

 negative attraction, i.e. a repellent, is in a small pro- 

 portion essential to all flowering plants. Some species 

 flourish on soils rich in calcium-carbonate, e.g. Anthyllis 

 Vulneraria L., and Ophrys muscifera Huds., the Fly 

 Orchis. Others restricted to calcareous soil in one dis- 

 trict, may appear calciphobe or indifferent in another; 

 while that lime-needing plants are not merely those that 

 prefer dry situations is shown by such species as 

 Phyteuma orbicular e L., which will flourish in the driest 



