64 ROOTS ABSORB ORGANIC SUBSTANCES ALSO 



substances introduced into the interior of the plant, are none of them 

 digested there and converted to the general purposes of food ? A state- 

 ment of two or three facts will afford a satisfactory reply to these several 

 questions. 



1°. "When plants are made to grow in infusions of madder the radicle 

 fibres are tinged of a red colour. 



2^. The flower of a white hyacinth becomes red after a few hours, 

 when the earth in which it is planted is sprinkled with the juice of the 

 jphytolaca decandra (Biol). 



Therefore organic substances can enter into the roots, and thence into 

 the circulation, of the plant. 



3°. The colour of the madder does not usually extend upwards to 

 the leaves and flowers of the plant. 



4°. The colour imparted to the flower of the white hyacinth disap- 

 pears in the sunshine in the course of a few days. 



Organic colouring mattenrs, therefore, undergo a chemical change either 

 in the stem, in the leaf, or in the flower — some sooner, some later — and 

 the same is probably the case with most other organic substances which 

 gain admission into tlie interior of plants. 



5°. Sir Humphry Davy introduced plants of mint into weak solutions 

 of sugar, gum, jelly, the tanning principle, &c., and found that they 

 grew vigorously in all oi^ them. He then watered separate spots of grass 

 with the same several solutions, and with common water, and found all 

 to thrive more than that to which common water was applied — while 

 those treated with sugar, gum, and gelatine grew luxuriantly. — [Davy's 

 Agricultural Chemistry, Lecture VI.] 



Therefore dilTerent organic substances — being introduced into the cir 

 culation and there changed — are converted by plants into their own sub- 

 stance, or. act as food, and nourish the plant. 



We may consider it, therefore, to be satisfactorily established that, 

 while a plajit sucks in by its leaves and roots much carbon in the form of 

 carbonic acid, it derives a variable portion of its immediate sustenance 

 (of its carbon) from the soluble organic substances that are within reach 

 of its roots. 



This fact is never doubted by the practical husbandman. It forms 

 the basis of many of his daily and most important operations, while 

 the results of these operations are further proofs of the fact. 



The nature of the soluble substances which are formed during the de- 

 cay of animal and vegetable substances — and which the roots of plants 

 are supposed to take up — will be considered in a subsequent lecture.* 



§ 3. Source of the hydrogen of plants. 



The source of the hydrogen of plants is less doubtful, and will re- 

 quire less illustration, than the source of the carbon. This elementary 

 substance is not known to exist in nature in an uncombined state, and, 

 therefore, it must, like carbon, enter into plants in union with some other 

 element. 



1°. Water has been already shewn to consist of hydrogen In combina- 



• Thia part of the subject might have been discussed here without appearing out of place 

 —but it will come in more appropriately, I think, when treating of the nature and mode of 

 aiJtion of vegetable manurea. 



