104 MANAGEMENT OF GREENHOUSE PLANTS. 



Having shosvn that air and water are essential to vegeta- 

 tion, and light to its colour, experience shows us that heat, 

 in a greater or less degree, is not less necessary to the growth 

 of plants; it is therefore requisite, that in taking plants into 

 our rooms, we should attend to these particulars. 



The internal structure of plants is composed of minute 

 and imperceptible pores, which serve the same important 

 purpose in the vegetable as veins in the animal system ; 

 they convey the circulation of the sap in the former, as the 

 veins do the blood in the latter ; but it is by no means 

 setileJ as yet by physiologists how the food of plants is taken 

 up into the system, and converted into their constituent 

 parts. 



From the foregoing considerations and facts, it is evident, 

 that, as air, heat, and moisture, are each essential to vegeta- 

 tion, that water should only be given in proportion as heat 

 and air are attainable. Tn the Summer season, greenhouse 

 plants may be exposed to the open air, from the early part 

 of May, until the end of September, by being placed on the 

 ledges of windows, or on a stand erected for the purpose, 

 or in the absence of a nursery bed of flowering plants, they 

 may be introduced into the regular flower-beds, to supply 

 the place of such plants as may wither and die in the course 

 of the Summer, by being turned out of the pots and planted, 

 or plunged in the earth with the pots. 



In the heat of the Summer season, plants generally 

 require water every evening, and in the absence of dews, 

 the earth about their roots may sometimes need a little in the 

 morning; but experience shows, that the roots of plants 

 more frequently get injured from being soddened with water, 

 than from being kept moderately dry. 



Having before intimated that exotic plants will generally 

 thrive best in a temperature and soil similar to that in 

 which nature first produced them, it may be necessary to 

 remind the reader, that we have the means of obtaining 

 suitable composts from our own soils, and from sand, 



