26 
NATURE NOTES 
the main or tap-root will have descended vertically, though the 
apex grows downwards in a spiral course. This accounts for 
the fact that two roots growing side by side are sometimes 
found to be twisted spirally together. Lateral or secondary 
roots are not so strongly influenced by gravity, and so grow 
out at various angles from the first. This is of course an 
advantage, so that the soil may be searched for water and 
nourishment in all directions. 
The origin of secondary rootlets should be noticed. With 
the aid of a simple microscope or strong pocket-lens they will 
be seen to issue from a deep-seated layer below the surface. 
This is called the pericycle. It is the same layer which gives 
rise to the fibrous tissue in many stems, as of flax and hemp. 
The secondary roots are consequently said to be “ endogenous,” 
i.e., “growing from within;” whereas branches on a stem are 
continuous with the latter and are “ exogenous,” or “ growing 
from without.” The way these roots issue is interesting, 
because instead of merely bursting through the cortical tissue 
of the “ mother-root,” they carry a “ digestive cap” at the end, 
which secretes a ferment. This dissolves the overlying tissue, 
which is now absorbed by and so nourishes the young root until 
it escapes externally. 
Circulation of Water. — If a young plant, say of wheat, 
be pulled up and gently washed it will be found impossible to 
remove all the particles of soil and minute fragments of stones 
adherent to it. The fact is that they are cemented to it, as 
the roots insinuating themselves adhere to the soil, in order to 
extract the film of water on the particles. The water then 
percolates through the cell-wall. This has no visible pores, 
but both animal and vegetable membranes are permeable to 
water. Thus, if a piece of bladder be stretched over an 
inverted funnel, which is to be filled with a solution of treacle 
or brown sugar and water, and then dipped into a vessel of 
pure water, the latter after a time will become darkened and 
sweet, while the water in the funnel will rise up into the neck. 
This process is called “osmose;” and may be described gener- 
ally — omitting causes of modifications with different kinds of 
membranes and fluids — as the rule that, if two fluids of different 
densities be separated by a membrane, diffusion takes place 
between them until equilibrium is established, the less dense 
fluid passing through the more rapidly of the two. 
This process of osmose takes place between the cells of 
vegetable structures and so helps to diffuse the fluid sap from 
cell to cell. The result is that the water of the soil which con- 
tains only about 2 or 3 per cent, of mineral matters passes more 
readily into the plant, and therefore causes great tension in 
the cells. This is relieved by the water passing on, inwards, 
from cell to cell, until the long open vessels are reached, when 
it readily passes up into the stem. 
Respiration. — This is another function of roots, as of all 
