260 HOW CROPS GROW. 
trees appears to result from wounds or disease, and the 
matters which in the experiments of Gyde and others 
were observed to be communicated by the roots of plants 
to pure water, probably came either from the continual 
pushing off of the tips of the rootlets by the interior 
growing point—a process always naturally accompanying 
the growth of roots—or from the disorganization of the 
absorbent root-hairs. 
Under certain circumstances, small quantities of mineral 
salts may indeed diffuse out of the root-cells into the water 
of the soil. This is, however, no physiological action, 
but a purely physical process. 
Vitality of Roots.—It appears that in case of most 
plants the roots cannot long continue their vitality if their 
connection with the leaves be interrupted, unless, indeed, 
they be‘kept at a winter temperature. Hence weeds may 
be effectually destroyed by cutting down their tops; al- 
though, in many cases, the process must be several times 
repeated before the result is attained. 
The roots of our root-crops, properly so-called, viz., 
beets, turnips, carrots, and parsnips, when harvested in au- 
tumn, contain the elements of a second year’s growth of 
stem, etc., in the form of a.bud at the crown of the root. 
If the crown be cut away from the root, the latter cannot 
vegetate, while the growth of the crown itself is not 
thereby prevented. 
‘As regards internal structure, the root closely resembles 
the stem, and what is stated of the latter on subsequent 
pages, applies in all essential points to the former. 
§ 2 
THE STEM. 
Shortly after the protrusion of the rootlet from a ger- 
minating seed, the Srem makes its appearance. It has, in 
general, an upward direction, which in many plants is per- 
