30 THEIE SUPPOSED BXCEETIONS. 



that such results are only obtained when roots are lacerated, 

 and that they have no greater power of excreting matter than 

 other parts of the surface of a plant. 



TUs theory of root excretions was sustained by Liebig, who regarded 

 excretion as the necessary result of secretion. It is now abandoned. 

 A correspondent of the Gardeners' Chronicle rightly observes in 

 answer to the question of what becomes of the inorganic matters which 

 plants are constantly taldng up? that in many instances, when taken 

 up in large quantities, they are deposited in the tissues themselves so 

 profusely as to obstruct the fnnetions necessary to the life of the plant, 

 and death is the consequence. Where these inorganic substances are 

 not taken up in sufficient quantities to destroy the life of the plant, 

 they are deposited in the tissues of the plant, either externally or 

 internaUy, or both, according to its structure. Plants growing on the 

 sea-shore, as Salsola and others, when exposed to the absorption of 

 large quantities of sea-water, deposit in great abundance crystals of 

 chloride of sodium in their tissues and upon their epidermis. He 

 has examined Gharas growing in pools, where the waters, from the 

 presence of carbonic acid, hold in solution great quantities of carbonate 

 of lime, and he has found this salt filling their large interceUular 

 cavities, and forming a crop of beautiful crystals on their epidermis, 

 •whUst those of the same species, growing in ponds with a less quantity 

 of carbonate of lime, have exhibited a comparative paucity of crystals. 

 The colouriag of wood, also, by introduoiag solutions of the metaUic 

 oxides into trees, is a good illustration of the mode in which superfluous 

 iaorganio matters are disposed of in the tissues of a plant. 



As to the excretion of organic matter, there is no need to limit that 

 function to roots, for nature assigns it to all parts of the surface, stems, 

 leaves, flowers, and fruit, as is seen by such familiar facts as honey- 

 dew, glandular discharges like those of the Sweet-Briar Rose, nectarial 

 emissions, &c. 



In general, roots have no buds, and are, therefore, incapable 

 of multiplying the plant to which they belong. But it con- 

 stantly happens, in some species, that they have the power of 

 forming what are called adventitious buds ; and, in such cases, 

 they may be employed for purposes of propagation. There is 

 no rule by which the power of a plant to generate such buds by 

 its roots can be judged of ; experiment is therefore necessary, 

 in all cases, to determine the point. 



Exceptions to the common rule are found in the Moutan Pseony, 

 in the Plum tree, or the Pyrus (Cydonia) japonica, which may be 



