40 



CASSELL'S POPULAE GARDEXIXG. 



TVarington, in his " Chemistry of the Farm," 

 states that "the carbon, hydi-ogen, and oxygen form 

 the cellulose, lignose, pectin, starch, sugar, fat, and 

 vegetable acids which plants contain. The same 

 •elements, imited vdth nitrogen, fonn the amides and 

 alkaloids ; and f ui-ther, united with sulphur, the still 

 -more important albuminoids, which are essential 

 <;onstituents of all plants." 



Sources of Plant-food. — Leaving out of con- 

 sideration such exceptional cases as those brought 

 to light in Darwin's beautiful investigation on ia- 

 sectivorous plants, and also the sources of the organic 

 substance of fungi, and, perhaps, of some forced 

 horticultui^al production?, it may be stated that 

 the source of the carbon of vegetation generally 

 is the carbonic acid existing, in very small -pvo- 

 portion, but in large actual amount, in the atmo- 

 sphere. 



The assimilation of carbon by plants is effected by 

 a very simple process. Carbonic dioxide, formed by 

 the union of carbon and oxygen, is absorbed by the 

 leaves of plants, in the substance of which it is 

 decomposed, the carbon being absorbed by the plant, 

 whilst the oxygen is set free. This succession of 

 effects is carried on without interruption, but yiith. 

 variable intensity, i;p to the time of flowering. 

 As soon as the flower is in bloom, and the young 

 .seed begins to develop, the growth of the plant 

 gi'adually lessens, and soon stops completely, and 

 the flower, instead of absorbing carbonic acid into 

 its substance and di-awing in light and heat, like 

 the foliage, absorbs oxygen, sets free carbonic acid, 

 and radiates heat. 



To convey a more definite idea of what is accom- 

 plished by vegetation ia the assimilation of carbon 

 from the atmosphere, may be quoted the results of 

 some du-ect experiments made by Sir J. B. Lawes 

 and Dr. J. H. Gilbert at Eothamsted. 



In a field, which has now grown wheat for forty 

 years in succession, there are some plots in which not 

 an ounce of carbon has been retm-ned to the land 

 during the whole period. Yet, with pui-ely mineral 

 manm-e, an average of about 1,000 lbs. of carbon is 

 annually removed, and where a given amount of 

 nitrogenous manm-e is employed with the mineral 

 manure, an average of about 1,500 lbs. per acre -per 

 annum more is obtained ; in all, an average of about 

 2,500 lbs. of carbon annually assimilated over an 

 acre of land without any return of carbonaceous 

 manure to it. 



The source of the hydrogen in plants is water ; 

 the water, being sucked up fi-om the soil by the 

 roots of the growing plants, is in part decomposed 

 within their cells, and the resultant elements com- 

 bine in some peculiar way with the carbon absorbed 



by the leaves to form sugar, starch, cellulose, and 

 the other proximate constituents. 



With regard to the sources of the element nitro- 

 gen, the explanations are by no means so simple. 

 The quantity of this element overlying every acre of 

 the earth's sui-face amounts to over 32,000 tons ; yet 

 it is probable that the majority of the plants culti- 

 vated in the garden or on the farm are incapable of 

 directly assimilating from this immense store of the 

 free element the nitrogen they require to build up 

 their structure during growth. This subject is now 

 occupying the attention of many eminent agricul- 

 tural chemists. 



Experience in both the field and the laboratory 

 has proved that cultivated plants obtain a portion 

 of their nitrogen in the fonn of ammonia, nitric 

 acid, and other nitrogenous compounds, from the soil 

 tlai'ough the medium of their roots. The product 

 of the putrefaction of one generation of plants and 

 animals thus affords a supply of available food for 

 its successors. 



Xitrogen, in combination with hydrogen, forms 

 ammonia, and the substance with which most horti- 

 culturists are acquainted as ammonium salts is 

 obtained from an extinct vegetation which probably 

 existed ages ago. 



GLASS STRUCTUEES AN^D 

 APPLIAJ^-CES. 



GEEEK-HOrSES AND CONSERVATORIES. 



THE chief distinction between a pit and a green- 

 house may be defined thus : the latter has a door 

 in the end or side for admission into the interior, and 

 a path a part or the whole of the way roimd it, and 

 the fomier has not. The difference between green- 

 houses and conservatories have also been thus de- 

 scribed : — In the green-house the plants are mostly 

 grown in pots and are portable, in conservatories 

 they are planted out and peirmanent. Xeither of 

 these definitions is anything like exhaustive or 

 complete. For we have sunk houses in plenty as 

 like pits as two peas are like each other, and deep 

 pits with paths, doors, kc, in plenty ; green-houses 

 also aboimd in which not a few of the plants, and all 

 the climbers, are planted out, as well as conserva- 

 tories in whiich all the plants are portable. 



Purpose rather than pots or planting-out has been 

 adopted by some as the ground of difference between 

 these two styles of houses. Green-houses have been 

 looked upon as places for growth and the safe storage 

 Oi plants, rather than theu' display for pleasure or 

 ornament ; and no doubt very many of them are so 



