ON ROCK PLANTS. 



289 



cases the soil in these crevices, or fissures, should be in connec- 

 tion with the bulk underneath, so that the roots may descend 

 to any depth. Vertical crevices should always be narrower at 

 the bottom than at the top, so that when the soil settles down 

 it will fall against the sides of the rocks without leaving hollows. 



A few alpines, such as the thick, fleshy-leaved Sempervivums, 

 will hold on almost to the bare rock and grow with very little 

 soil, but these are the exception : the majority prefer a deep-rooting 

 medium, so that the roots may run down by the sides of the 

 rocks to a good depth ; they are thereby kept cool and moist, and 

 are better able to withstand the occasional droughts experienced 

 during our variable summers. Many of them are not fastidious 

 as to soil, and a good turfy loam intermixed with plenty of 

 smashed sandstone to keep it porous will be found suitable for 

 the majority of kinds. Of course those requiring special mixtures 

 can easily be supplied by filling up the spaces set apart for them 

 with the kind of soil needed, instead of with the ordinary soil. 



Great difficulty is often experienced in keeping through the 

 winter plants which have their leaves covered with " down," such 

 as the silky-leaved Androsaces, for the water collects in the down 

 and rots them away. With such subjects it is a good plan 

 to plant them so as to be overhung by ledges of rock; or they 

 might be planted in a dry, airy situation, and have squares of glass 

 elevated over them during the winter, so as to keep off the rain. 

 The majority of alpines prefer a position fully exposed to the sun ; 

 such situations suit the rare and minute species, whilst a stony 

 surface is also beneficial, for the particles of stone prevent 

 excessive evaporation and tend to keep the roots cool and moist. 

 A well-constructed rockery should provide all aspects, so that 

 both sun- and shade-loving plants may be accommodated. A 

 knowledge of the various plants and their requirements is essential 

 before suitable positions can be chosen for them. A great point 

 is to insert each firmly and right up to the collar of the plant. 



When once planted, the after-management is simple enough, 

 and consists mainly in keeping free from weeds, in preventing 

 the strong-growing kinds from overrunning and smothering the 

 weaker and more diminutive ones, and in giving them an 

 abundance of water during hot dry weather in the summer. 

 Slugs and mice must also be trapped, or many of the delicate 

 little alpines will soon disappear. Plants which raise their collars 

 will require top-dressing or replanting annually or they will soon 

 become leggy, as is the case with many of the Primulas. 



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