110 BOTANY. 
’ * 
mass called a spongiole or little sponge, and by these cells the fluid 
nourishment of the plant is imbibed. Animal and vegetable membranes 
have the property of allowing fluids to pass through them, not by any 
special pores, but at all parts, through minute interstices in their whole 
structure ; and when the membrane is very thin, as that is which forms 
the cells e a spongiole, and the fluid on one side of it differs considerably 
in density from that on the other, an exchange pretty rapidly takes place 
by what is called osmotic action.1 This may be exemplified by filling a 
bladder with brine from salted meat, and placing it in a vessel full of 
pure water, when much of the salt of the brine will soon be found to 
pass through the bladder into the water by which it is surrounded. The 
animal substances contained in the brine will, however, remain in the 
bladder, which will receive a quantity of water-greater than that of the 
substances which have passed out. Osmotic action is carried on to a 
great extent in nature, the cells both of animals and plants thus 
imparting their contents to. those which are in contact with them, so 
that a motion of fluids is kept up even where there are no proper 
veins or tubes, and changes are at the same time effected in the fluids 
themselves. And thus it is that the roots of plants take in nutriment, 
whilst they are also continually giving forth substances which the plant 
may be said to desire to be quit of, and the continual growth of the root 
pushes forward the spongiole into a new place, where it finds fresh soil. 
The soil in which a plant grows therefore undergoes a twofold change, 
being deprived of the substances most beneficial for the nourishment of 
the plant, and more or less filled with others which are specially unsuit- 
able to it ; but these are often very suitable for other plants; and thus 
the farmer finds a rotation of crops more advantageous than a con- 
tinual repetition of the same kind of crop on the same land, 
Roots are covered with a bark like that of stems ; and the roots of some 
plants, if by accident or design exposed to the air, produce buds from 
which shoots or branches proceed. The roots of many plants extend 
underground and send up shoots. Many plants also send forth roots 
from the stem or branches, when these are brought into contact with the 
soil, or partially buried in it ; and advantage is taken of this to propagate 
some kinds of trees and shrubs by cuttings, which soon produce roots 
when planted in moist earth, or by layers, which are branches bent down 
and partially covered with earth, but not separated from the parent 
plant till the new roots are formed. Some plants, in their ordinary 
growth, send forth roots from the stem or branches, which, as they 
elongate, seek the ground, and form new stems or props to the stem, 
adding also to the supply of nourishment from the soil. A remarkable 
1 From Greek cosmos, impulsion, 
“ “ 4 ee iat = he eae aE my bi 
a Nae ee Ne egies 
oe 
a el et eat, near alial i 
