PRINCIPLES OF GARDENING. 49 



atmosphere ; and the roots must be in contact with soil in a particular 

 physical condition. Some plants, as orchids and ferns, must be kept in 

 a moist atmosphere; others, as cacti, like a drier condition. Some plants, 

 as vines, like a moist atmosphere whilst they are growing, and a drier 

 atmosphere whilst they are fruiting ; and in my garden there are many 

 contrivances for supplying water to the air of the glass-houses. 



The amount of woody fibre, which may be reduced by leaves 

 from the carbonic acid of the atmosphere, has been determined by 

 Messrs. Lawes and Gilbert to be about 4,000 Ibs. in weight in a 

 single year; and Professor Odling has computed the air over one 

 acre of land to contain 20,000 Ibs. of carbon in a state of com- 

 bination as carbonic acid, which is present to the extent of about 2\ 

 per cent. 



The roots of a plant must have their own proper position. 

 The orchids or air-plants have their roots in the air, or merely 

 covered with moss. Some plants like peaty soil ; others, as peach- 

 trees, require the closest loam. Fruit-trees in pots will not succeed 

 unless the soil is rammed with a mallet to make it close. The 

 Riuncx aquaticus flourishes with its roots under water, whereas most 

 trees, and numerous plants, would perish if their roots were under 

 such conditions. The surface of a pot is favourable for the roots of 

 many plants ; as they derive air, moisture, and probably earthy salts, 

 in such a situation. Brick rubbish is a very suitable material 

 for the roots of a great multitude of plants. 



The three principal soils used in my garden are top spit loam from 

 commons, coarse sand, and fibrous peat, which is entirely a vegetable 

 product. In these materials, alone or mixed together, the greater 

 number of all plants may be grown. At Florence I learnt that rotten 

 wood took the place of peat for camellias and azaleas, and on trial 

 in this country I found it to succeed perfectly. 



The supply of water to plants is necessary for vegetation. It is 

 mainly supplied by the root ; and for every grain of solid constituents 

 added to a plant it is found, by experiment, that about 250 grains of 

 water must pass through it. It is fatal to many plants to move 



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