220 SIKKIM RHODODENDRONS. 



The notorious difficulty o£ cultivating some of the Sikkim 

 Rhododendrons is probably to be explained by similar consi- 

 derations. In the Sikkim Himalaya there is little continued 

 sunshine, and the warm air is perpetually charged with 

 moisture brought by the southerly winds, which discharge it to 

 the extent of 120 to 140 inches annually {Hooker). There 

 appears to be no means of imitating such a state of things 

 artificially. We may secure the temperature, and we may load 

 warm air with vapour, but when we attempt to set the latter in 

 active motion, the humidity disappears. 



To the reasons already offered for regarding free ventilation 

 as a necessary condition of high cultivation, may be added the 

 importance of removing deleterious matters, such as sulphu- 

 rous acid, or of diluting those which, like carbonate of 

 ammonia, sulphuretted hydrogen, hydrochloric acid, &c. are 

 only wholesome when administered in quantities almost if not 

 entirely inappreciable by the senses. 



We know too little of the effects of volatilised corrosive sublimate, to 

 say whether free ventUation would not prevent the destructive action 

 of that agent upon plants kept in greenhouses built of Kyanised wood. 



It has been thought that an irresistible argument against 

 the need of free ventilation is to be found in the moveable 

 structures called "Waedian Cases, in which it is alleged that 

 plants grow luxuriantly although cut off as far as possible 

 from aU access to air. But in reality such contrivances are 

 not at all suited for the cultivation of plants ; they are only 

 applicable to their preservation for limited periods of time. It 

 was the success that attended the latter which led gardeners to 

 suppose Ward's Cases equally well suited to the former, and 

 to fall into the error of disbelieving in the necessity of free 

 ventilation. 



As the Wardian Case is largely employed in Horticulture, especially 

 in the decoration of sitting-rooms, it seems desirable to point out in 

 this place what are its real merits and defects. 



When Mr. Ward first remarked a Grass and a Moss growing inside a 

 damp bottle, he merely saw what gardeners had witnessed for a couple 

 of centuries at least. He beheld the propagator's bell-glass with its 



