130 
ROCKS AND ROCK-PLANTS. 
tank of water ; in the concealment of some unavoidable and unsightly feature ; 
forming an irregular slope against the back of a house with a lean-to roof ; or in 
a span-roofed house to conceal the parapet- wall in the place of a shelf. As, in the 
last case, it would occupy the place commonly allotted to the heating apparatus, 
the floor should be constructed of slabs of wood about four inches broad, and a 
space left beneath for the pipes. By this arrangement the latter would be hidden 
from the eye ; and they would communicate a degree of bottom-heat to the 
rockery, if a little care were employed in its formation. So conditioned, we 
entertain little doubt that it would be favourable to the cultivation of many of 
those orchidaceous plants, which in their native haunts cling to rocks and stones. 
Cisterns for water are necessary appendages to all erections for the manage- 
ment of plants ; but they are often awkward and disagreeable objects. This 
seems to have been so far considered as to lead to the adoption of various modes of 
hiding them ; such as, placing them in a back shed, communicating with the 
house by means of a pipe and tap, and fixing them in the wall and covering in 
different ways. All these, however, are open to objection. "Water that has been 
exposed to the influence of the sun is more favourable to vegetation than that 
which has been kept from it. Professors Morren have stated that this is owing to 
the greater quantity of oxygen which it holds in solution ; and that this 
oxygenation is determined by the action of solar light on the respiration of plants 
and animalcules : hence, one reason also why it is better to water in the afternoon 
of a sunny day, than in the morning. Now, by shelving off the upper part of 
the sides of a cistern, and forming a small rockery round the margin, of some kind 
of stone possessing a rough uneven surface, to be planted with suitable species, we 
should have an object that would be interesting in the most conspicuous station. 
For small piles of this nature, the stone called tufa, obtained in various parts 
of Derbyshire, especially about Matlock, is perhaps the most suitable that can be 
selected, both from its peculiar irregular surface and comparative freedom from 
harsh angles, and because most plants seem to delight in growing upon it. This 
material has been employed largely for rockwork in the pleasure-grounds at 
Elvaston Castle, in Derbyshire, the seat of the Earl of Harrington ; and, though 
too expensive to procure to be employed commonly in extensive works, is decidedly 
of a nature most fitting for the purpose in conservatories and other glass-houses. 
Any limestone, or other stone witli a similar surface, may be employed. 
We would caution our readers, however, against the absurd practice of 
mingling together many different kinds of stone, with the view of thus con- 
tributing something conspicuous. As we have said before, the rock ought to be 
the subordinate feature ; the plants which grow upon it are to be looked to for the 
principal ornament. 
In the larger description of horticultural buildings, rockeries may be constructed 
on a more extensive scale, and independently of the restrictions to which it is 
necessary to confine them in small houses : to a piece of water they are almost a 
