18 THE FLOWERING PLANT. 
and root-hairs, by means of osmosis, much as described on p. 9. 
The particles of soil are also more or less corroded by the acid sap 
of the root-hairs, &ec., to which they closely adhere (fig. 3). This 
permeates the cell-walls with which the particles 
come into close connection. If the roots of a 
plant are allowed to grow over a polished slab 
of marble, this will be corroded at the points of 
contact, and a kind of rough etching of the 
root system produced. Roots thus help in the 
preparation of the solutions which they absorb. 
This solvent action is aided by the carbon 
dioxide excreted in respiration. The roots of 
. water plants can easily avail themselves of the 
surrounding medium with the substances dis- 
solved in it. Parasitic roots come into such 
close relation with the tissues of the plant preyed 
upon, that they form a physiological part of it, 
and can by its means receive not only simple 
food, but also material that can at once be built 
up into protoplasm. 
Circulation of Liquids.—The substances ab- 
sorbed by the root ultimately reach the leaves 
by various routes and in various ways. Just as 
they can enter the root-hairs and young epider- 
mal cells by means of osmosis, so can they in 
the same way reach cells that are more deeply 
situated, and so on. We also know that liquids 
Peed ede ae travel very largely by means of certain vessels 
WhiteMustard[from (¢/ p. 41), chiefly,as one would expect, in their 
Sars}. A. as taken interior. This has been noticed for a very long 
particles of earth time in the case of vines. When these are 
funging to the root- pruned in spring, a great deal of sap exudes 
hairs; in B_ these 
have been removed from the cut surfaces. In popular language, 
by washing in water. oa ” 7 7 
Above are seen the the plant ‘‘ bleeds,” and careful examination 
two green expanded shows that the liquid oozes out from the 
cotyledons; between Sy : 
these and the root- cavities of the vessels. This phenomenon was 
hairs, the hypocotyl. formerly ascribed to a mysterious force called 
root pressure, operating before the leaves unfold. It was 
erroneously supposed that at other times liquids travel only in 
the walls of the vessels, and not within them. 
Roots may perform another important office in nutrition, ze., 
the storage of reserve materials, which are supplies of nutriment 
destined for use at some future time. The matter is chiefly stored 
as starch, but it may assume other forms, ¢.g., cane-sugar, as in 
the beet-root, 
NV 
Di Uy SiN) Ry hy 
(A) 
