me thing in the nature of the soil or the weather retards them. If
the locality is cold, those plants which are propagated in the nursery
and are tender by nature ought to be protected from the frosts by
coverings of leaves or straw, and, if rains follow, care should be
taken that water is not permitted to stand any where about them, for
ice is a poison to tender roots under ground, as to sprouts above, and
prevents them from developing normally. In autumn and winter the
roots develop more than does the leaf of the plant because they are
nourished by the warmth of the roof of earth, while the leaf above is
cut down by the frosty air. We can learn this by observation of the
wild vegetation which grows without the intervention of man, for the
roots grow more rapidly than that which springs from them, but only so
far as they are actuated by the rays of the sun. There are two causes
of the growth of roots, the vitality of the root itself by which
nature drives it forward, and the quality of the soil which yields a
passage more easily in some conditions than in others.

_Of the mechanical action of plants_

XLVI. In their effect upon plants such natural forces as I have
mentioned produce some curious mechanical results. Thus it is possible
to determine the time of the year from the motion of the leaves of
certain trees like the olive, the white poplar and the willow, for
when the summer solstice has arrived their leaves turn over. Not less
curious is the habit of that flower which is called the heliotrope,
which in the morning looks upon the rising sun and, following its
journey to its setting, never turns away its face.

_Of the protection of nurseries and meadows_

XLVII. Those plants, which, like olives and figs, are grown in the
nursery from cuttings and are of a tender nature, should be protected
by sheds built of two planks fastened at each end: moreover they
should be weeded, and this should be done while the weeds are still
young, for after they have become dry they offer resistance, and more
readily break off in your hand than yield to your pull. On the other
hand the grass which springs in the meadows and gives you hope of
forage not only should not be rooted out while it is growing, but
should not even be walked upon; hence both the flock and the herd
should be excluded from the meadow at this time and even man himself
should keep away, for grass disappears under the foot and the track
soon becomes a path.