154 GARDEN PLANNING 



these natural conditions, and if it fails in this, 

 its principal function, how can it serve its pur- 

 pose as a home for the plant? 



The idea, not by any means uncommon with 

 those who have not looked closely into the 

 question, that there is some magic virtue in 

 the rock itself by which these plants obtain 

 food suitable for their needs, must be aban- 

 doned. The function of the rock pieces in 

 our alpine garden is twofold. They serve to 

 give us a suggestion of the natural environment 

 of the plants we wish to grow, which is good; 

 but, better still, they conserve the moisture in 

 the soil, and thus ensure that the plants have 

 a constant supply of it in all weathers. 



This being so, it is well to select our rocks 

 from material which is porous, and therefore 

 capable of holding in its substance a certain 

 reserve of moisture. Yet an impervious stone 

 is better than none, as it reduces the area of 

 soil subject to evaporation. 



The next point to understand is that the soil 

 in our rock garden must be well drained. This 

 may be attained by ensuring that it is of a 

 porous character and of sufficient depth. Drain- 

 age is assisted by elevating the rock garden, 



