156 REMARKS ON GROWING 1>LANTS IN GLASS CASES. 



show that degrees of cold may be borne with impunity in an 

 atmosphere that is perfectly still, which, if accompanied with a brisk 

 wind, would be quite intolerable. That such stillness prevails in 

 the plant cases there can be little reason to doubt ; for though con- 

 siderable motion may often occur in the air within them from varia- 

 tions in the external heat, yet, as little or none of this air escapes, its 

 temperature, at any given period, must be deemed pretty uniform, 

 and cannot be rapidly reduced as it is by the frequent contracts and 

 changes of surface which go on in the free motions of an agitated 

 atmosphere. In an atmosphere, too, which is so still, and in which 

 changes of temperature proceed so slowly, tropical plants may, 

 perhaps, bear, without injury, degrees of cold fvhich would prove 

 fatal if occasioned by the frigorific operation of free air in constant 

 and rapid motion. 



Condition of Plants, with regard to Light, in close Cases and in 

 the free Atmosphere. 



Of the great importance of light to vegetation Mr. Ward is fully 

 sensible. " The success of his plan," he says, " will be in propor- 

 tion to the admission of light to all parts of the growing plants. In 

 every place," he adds, " where there is light, even in the centre of 

 the most crowded and smoky cities, plants of almost every family 

 may be grown " by this method. Seeds, it is well known, germinate 

 best when buried in the soil, and entirely secluded from light; but 

 when the young germ pushes into day, if light be still excluded, by 

 inverting over it an opaque vessel, the plant shoots up into a long 

 and feeble stem, is of a pale or whitish-yellow colour, and possesses 

 little odour or savour. On the contrary, if the vessel be transparent, 

 so as to transmit light, the growth is more vigorous, the young plant 

 puts forth buds from its stem, and soon exhibits its characteristic 

 form and colour. 



These differences in form and colour, according as plants are 

 grown in light or in darkness, were early noticed by Ray, and after- 

 wards by M. Bonnet, in his " Recherches sur l'Usage des Feuilles," 

 p. 210. In the year 1771 Dr. Irvine described still more minutely 

 the influence which light exerts on vegetation. " Plants," says he, 

 " though furnished with water, heat, and air, grow imperfectly, if 

 placed in a dark box, and never contain anything but a watery juice ; 



