792 EXCRETION BY ROOTS. 
press the result produced by virtue of the mutual actions of the 
root and the substances which surround it in the soil. This 
power or property of selection is without doubt due to some at 
present but little understood molecular relation which exists 
between the membranes of the cells of different plants and the 
substances which are taken up or rejected by them, different 
roots possessing different osmotic action for the same substances. 
It follows also, from the recognition of this action as the cause 
of the absorpt on of fluid matters by the plant, that poisonous 
substances may be taken up when in solution by the roots, pro- 
vided their tissues are not injured by them in their passage ; 
and we find, accordingly, that when such substances are found 
in the soil, a corresponding effect is produced upon plants by 
their absorption. 
Excretion by Roots.—Roots seem to have no power of getting 
rid of excrementitious matters like that possessed by animals ; 
but that they do throw off into the soila portion of their contents 
by a process of exos- 
mose, which appears 
to be an almost neces- 
sary result and ac- 
companiment of the 
endosmose by which 
absorption takes 
place, is possible. 
Carbon dioxide, and 
possibly other acid 
substances, are parted 
with by roots in this 
way ; and thus assist 
subsequent absorp- 
Fig. 1144, Longitudinal section through root of Preris tion vy dissolving 
hastata, showing apical region. v. Apical cell, from substances which 
which are developed the tissue of the substance of eould not otherwise 
the root, 0, c, and the root-cap or pileorhiza, x, 7, m, n. . 
(See page 125.) (After Sachs.) P= mto the plant. 
This is proved by 
Sachs’ experiment of letting roots grow over a slab of polished 
marble, which was eroded wherever the roots came in contact 
with it. 
Storing of Nutriment by Roots Roots are frequently en- 
larged by becoming reservoirs of nutriment in the form of 
starchy, gummy, and similar matters for the future support of 
the plant. The tubercules of the Dahlia (jig. 263) and Orchis 
(figs. 261 and 262); and the roots of the Turnip (fig. 269), 
Carrot (fig. 267), and other biennials, are fainiliar illustrations. 
Development of Roots.—The growing part of the root is called 
the growing point (punctum vegetations). Itis commonly spoken 
of as the apex of the root, but is not really so, since it is covered 
with a cap of cells, the pileorhiza. (See pages 125 and 126.) 
Fic. 1144. 
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F 
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7 
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4 
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