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, &c., 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 
PE ae gy travel very largely by means of certain vessels 
WhiteMustard{trom (cf. p. 41), chiefly along their walls, not, as one 
Sachs), A. as taken would expect, in their interior. In the spring, 
out of the soil with 
particles of earth however, before the leaves have unfolded, ab- 
pinging to she 1° sorption by the roots is so active that liquid is 
have been removed forced into the interior of the vessels, and so 
by washing in water. me i 5 
Above are seen the Upwards. ‘This phenomenon is known as root 
two green expanded »resswre. It has been noticed for a very long 
cotyledons; between +, . 2 
these and the root- time in the case of vines. When these are 
hairs, the hypocotyl. runed in spring, a great deal of sap exudes 
from the cut surfaces. In popular language, the plant “ bleeds.” 
Careful examination shows that the liquid oozes out from the 
cavities of the vessels. This bleeding is the result of root pressure. 
Roots may perform another important office in nutrition, @e., 
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, e.g., cane-sugar, as In 
the beet-root. 
