80 
CONSTRUCTION OF HOT-HOUSE ROOFS. 
Who does not know that, after a hot dry day, the common Fracvinella emits 
a resinous vapour, which will readily take fire, and burn freely, if a candle be 
introduced to it, without destroying the plant. 
But leaving the subject of the fly-catching properties of plants ; in viewing 
their peculiarities and metamorphoses the mind is led to things exceedingly curious. 
Many of the movements of plants are purely mechanical ; others cannot be 
attributed to mechanism, because they are attended with phenomena exactly 
resembling muscular contractility in animals. 
The Hedysariim gyrans is constantly in motion. These motions are thought 
to be connected with respiration. In the tiger-lily the pistillum will bend first 
towards one stamen and then towards another, until it has received the pollen from 
nearly all. In Saxifraga and Parnassia something of the same motion is observed 
with the stamina. The sensitive plant is very remarkable in its motions. The 
phenomena of sleeping and waking of many plants are well deserving of notice. 
M. Dutrochet has made some experiments, by way of ascertaining the real cause of 
this ; and his observations have led him to the conclusion that the sleeping, waking, 
and nutation of plants, depend on the air contained in their air-organs, and take 
place in proportion to the quantity of that air. 
OBSERVATIONS ON THE CONSTRUCTION OF ■ 
HOT-HOUSE ROOFS, M 
WITH A DESCRIPTION AND PLAN OF A GREEN-HOUSE ERECTED AT CHATSWORTH ; AL^H 
A SELECTION OF GREEN-HOLTSE CREEPERS, WITH REMARKS UPON THEM. 
For these few years past we have directed particular attention to the construc- 
tion of all sorts of hot-houses and green-houses. In doing so, we have always had j 
four things in view — namely, utility, stability, convenience, and, though last not |l 
least, economy. j 
Before we proceed to detail the different plans we have pursued, it may be well 
to mention the motives which induced us to commence the inquiry. For many 
years past it has been a bone of contention among practical men, as well as 
theorists, of what materials the roofs of hot-houses ought to consist. The advo- 
cates for metal contend that the advantages are great, when compared with wood^ 
both in lightness and stability. Those who disapprove of these kind of roofs;, 
object on account of the contraction and expansion of the metal, the difficulty, at 
certain seasons, in keeping them warm, and also of properly ventilating them ; 
and others object only on account of the expense. j 
For our own parts, the expense has been one, and, we may say, the chief 
objection ; for Avhile we admit they are more difficult to keep warm in winter, and 
sometimes troublesome in summer, on account of their liability to become very hot 
rather suddenly, yet we have never been blind to the great advantage they possess 
with regard to light. 
