44 



The moisture of the atmosphere, then must not be 

 neglected ; not only because the healthy action of the vital 

 organs of the plants depends on a proper hygrometrical 

 state of the atmosphere, but, inasmuch as it is the readiest 

 means both of avoiding, and when unhappily, they are 

 present, of destroying, many of the most destructive and 

 troublesome insect enemies, to whose depredations, plants 

 are subject. 



When a moist atmosphere is duly and regularly main- 

 tained, there is but little fear need be entertained of the 

 establishment of a colony of insects such as the thrip, 

 and the red spider, which are perhaps the greatest pests 

 which have to be overcome in the forcing house ; nor is 

 there a more effectual method of destroying them, than 

 by applying a high temperature in conjunction with an 

 intense degree of moisture. To the want of a balance 

 of moisture in the composition of the atmosphere, and in 

 the soil, too, rather than as is commonly supposed, to an 

 excess of it in the former, is the appearance called mildew 

 to be attributed; this it occasions by checking the regu- 

 lar action of the perspiratory organs, and thereby inducing 

 an eruption of the cells of the tissue : the extravasated 

 sap lodging on the cuticle, affords a nidus for the germi- 

 nation of the sporules of that particular fungus, which 

 when grown, is the mildew: the remedy consists in 

 avoiding an irregular composition of the atmosphere, as 

 regards heat and moisture; and also an excess or defi- 

 ciency of moisture in the soil, so that each may be in a 

 condition to exert its proper influence on the constitution 

 and developement of the plants. Canker, another dis- 

 ease, to which Cucumbers are sometimes subject, appears 

 to be produced by too low a degree of temperature, accom- 

 panied by an excess of moisture, both in the soil and the 

 atmosphere, and it generally attacks those particular 

 parts, where any check or obstruction is offered to the 

 flow of the sap, such as that occasioned by a wound, or 



