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 
a span-roofed house to conceal the parapet-wall in the place of a shelf. As, in tl! 
last case, it would occupy tlie place commonly allotted to the heating apparatrl 
the floor should be constructed of slabs of wood about four inches broad, and 
space left beneath for the pipes. By this arrangement the latter would be hiddi 
from the eye ; and they would communicate a degree of bottom-heat to t' 
rockery, if a little care were employed in its formation. So conditioned, \ 
entertain little doubt that it would be favourable to the cultivation of many '* 
those orchidaceous plants, which in their native haunts cling to rocks and stones. 
Cisterns for water are necessary appendages to all erections for the mana^' 
ment of plants ; but they are often awkward and disagreeable objects. Tl; 
seems to have been so far considered as to lead to the adoption of various modes ' 
hiding them ; such as, placing them in a back shed, communicating with tji 
house by means of a pipe and tap, and fixing them in the wall and covering , 
different w’ays. All these, however, are open to objection. Water that has beii 
exposed to the influence of the sun is more favourable to vegetation than th 
which has been kept from it. Professors Morren have stated that this is owingi 
the greater quantity of oxygen which it holds in solution ; and that tl 
oxygenation is determined by the action of solar light on the respiration of plai||l 
and animalcules : hence, one reason also why it is better to water in the afternojii 
of a sunny day, than in the morning. Now, by shelving off the upper partf 
the sides of a cistern, and forming a small rockery round the margin, of some kil 
of stone possessing a rough uneven surface, to be planted with suitable species, k 
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 pa|3 
of Derbyshire, especially about Matlock, is perhaps the most suitable that canp 
selected, both from its peculiar irregular surface and comparative freedom frji 
harsh angles, and because most plants seem to delight in growing upon it. T|s 
material has been employed largely for rock work in the pleasure-grounds t 
Elvaston Castle, in Derbyshire, the seat of the Earl of Harrington ; and, thoijji 
too expensive to procure to be employed commonly in extensive works, is decidey 
of a nature most fitting for the purpose in conservatories and other glass-housi. 
Any limestone, or other stone with a similar surface, may be employed. 
W e would caution our readers, however, against the absurd practice f 
mingling together many different kinds of stone, with the view of thus cl- 
tributing something conspicuous. As we have said before, the rock ought to|e 
the subordinate feature ; the plants which grow upon it are to be looked to for e 
principal ornament. 
In the larger description of horticultural buildings, rockeries may be construcjd 
on a more extensive scale, and independently of the restrictions to which i.is 
necessary to confine them in small houses : to a piece of water they are almoi'a 
