« Frey 
ABSORPTION BY ROOTS.—SELECTION OF FOOD. 791 
already mentioned (page 126), the cells at the extreme apex of 
the rootlets forming the cap are not adapted for absorption. 
Roots absorb more water than the plant requires, and this 
excess of fluid exerts a pressure up the stem called Root-presswre, 
which may be measured by cutting off the upper part of the 
stem of a growing plant and attaching a manometer to the cut 
end. (See page 822.) 
Roots, as will be shown (page 792), only grow in length 
by additions near to their extremities, and as it is at these 
parts that absorption of food almost entirely takes place, they 
are always placed in the most favourable circumstances for ob- 
taining it, because in their growth they are constantly entering 
new soil, and hence, as one portion of that soil has its nutritious 
matters extracted, another is entered which is in an unexhausted 
state. It has also been shown, by direct experiment, that when 
the roots meet with a store of nourishment in the soil, a greatly 
increased development of rootlets takes place for its absorption. 
Roots can only absorb substances in a liquid state, therefore 
the different inorganic substances which are derived from the 
soil, and which form an essential part of the food of plants, must 
be previously dissolved in water. If the roots of a freely 
growing plant be placed in water in-which charcoal in the most 
minute state of division has been put, as that substance is in- 
soluble in the fluid, it will remain on the surface of the roots, 
and the water alone will pass into them. 
Selection of Food by Roots.—Various experiments have been 
devised to ascertain whether the plant possesses any power of 
selecting food by its roots. Saussure proved, that when the 
roots of plants were put into mixed solutions of various salts, 
some were taken up more freely than others. He also found 
that dead or diseased roots absorbed differently to those in a 
living and healthy condition. The experiments of Daubeny, 
Trinchinetti, and others, lead essentially to the same conclusions. 
Again, though the seeds of the common bean and wheat be 
sown in the same soil, and exposed, as far as possible, to the 
same influences in their after-growth and development, yet 
chemical analysis shows that the wheat stalk contains a much 
larger proportion of silica (which it must have obtained from the 
soil) than that of the bean. 
The experiments of Bouchardat, Vogel, and others, appear, 
on the contrary, to indicate that roots absorb all substances pre- 
sented to them indifferently, and in equal proportions. But 
the simple fact, as just mentioned, which is easily proved by 
chemical analysis—that the ashes of different plants grown in 
the same soil, contain different substances or in different pro- 
portions—seems to prove incontestably that roots have a power 
of selecting their food. In using the term selecting, we do not, 
however, intend to imply that roots have any inherent vital 
power of selection resembling animal volition, but only to ex- 
