THE ROOT. 19 
Like all other parts of the plants, roots carry on Respiration. 
A supply of free oxygen is therefore necessary. - In the case of 
aquatic plants this is dissolved in the surrounding water, while in 
land plants air is present in the interstices between the particles 
of soil, and enters the plant dissolved in the cell-sap. It is 
evident, therefore, that if these interstices get completely filled 
with water, the roots cannot respire properly (since, unlike water 
roots, they are not adapted for utilizing oxygen dissolved in water) ; 
hence the sickliness of overwatered pot plants. Carbon dioxide, 
of course, diffuses out of roots in respiration, and, as mentioned 
previously, helps to bring the food into solution. 
Circulation of Gases.— Within the root gases can circulate as 
well as liquids. In fact, the cell-sap contains gases dissolved in 
it, which, of course, participate in its movements. ‘There are, 
however, special channels for the circulation of gases, viz., (1.) 
the cavities of certain vessels, which, except in spring, contain no 
liquid, and (2.) intercellular spaces, which are chinks or crevices 
that exist between the cells of parenchyma and some other tissues, 
and which form a set of irregular but communicating channels. 
It is comparatively seldom that Reproduction is effected by 
the root, and when this is the case, it is always vegetative (cf. p. 43), 
z.€., by development of ordinary leafy shoots; and not of special 
reproductive organs. One of the best examples is the common 
acacia (Robinia pseudacacia) of gardens, the roots of which spread 
horizontally and send up shoots that become new trees. Another 
case is that of the dahlia, which can be propagated from its root- 
tubers. 
Improbable though it may seem, the root exhibits a consider- 
able amount of Motility. Protoplasmic movements occur in some 
root-hairs, and young growing root-tips are in constant slow 
movement from side to side, forcing their way between the particles 
of soil. The older parts of the root move in a different manner, 
7.e., they shorten, and this causes the plant to be anchored very 
firmly in the ground. Rosettes of leaves like those of dandelion 
are thus prevented from being raised by growth of the stem. It 
has been remarked that a root burrows in the soil as an earth- 
worm does. The narrow anterior end of this animal makes its 
way between the particles of earth; the rest of the body then 
shortens and is pulled up to it. 
Irritability of the root is shown by its sensitiveness to various 
stimuli, such as gravity, light, moisture, and contact. Spontaneity 
is seen in the persistence of its movements. Geotropism refers to 
the influence of gravity in determining the direction of growth. 
Roots are positively geotropic, 7.e., grow towards the earth’s centre. 
The primary root takes a vertical direction, and the lateral ones 
