96 JOUENAL OF THE ROYAL HORTICULTUEAL SOCIETY. 
THE MOVEMENTS OF PLANT ORGANS. 
Lecture at Chiswick Gardens. 
By Eev. Prof. G. Henslow, M.A., Y.M.H., Sec. 
[June 14, 181)9.] 
The old distinction between plants and animals, that the latter can move, 
but the former cannot, has long since been abandoned as unscientific ; 
for probably all plants, even when permanently fixed to the soil, have 
their stems, leaves, flower-stalks, &c., in almost perpetual motion. 
Germination. — The first movement observable is in the tendency of 
the radicle to turn earthwards. As this is the case at all points of the 
globe it is attributed to the force of gravity, but its action is confined to 
the " growing point " immediately behind the apex protected by the root- 
cap. In the Cabbage the sensitive part consists of -02 to '03 of an inch, 
for it was found by Darwin that the influence was conveyed — doubtless 
by the continuity of the protoplasm from cell to cell — to a point some 
distance back ; for when a radicle is placed horizontally, though it soon 
curves earthwards, the point of curvature is behind the actual growing 
point. 
Besides this movement radicles circumnutate, or " bow around," as the 
w^ord means, thus finding the line of least resistance for entering the 
soil. 
The plumule rises upwards in opposition to gravity, originally under 
the influence of light ; but it has become so fixed in the constitution that 
seedlings will rise up vertically, even in total darkness, in opposition to 
gravity. But that light is the cause was proved by Mohl, who sowed 
seeds in earth on a gauze shelf placed near the bottom of an inverted box 
which was illuminated by a mirror from below. They now grew down- 
wards, both in the direction of light as well as of gravity. 
Roots. — Though the primary or tap root grows vertically downwards, 
the secondary roots, w^hich issue from the deep-seated layer called the 
pericycle, and emerge at right angles to the surface, are inclined at various 
angles to the vertical, and do not turn into the perpendicular direction 
unless the primary root be cut away. The object of searching the ground 
in every direction for moisture and food is thus secured. 
The expression " searching " is metaphorical, for the real process is due 
to the sensitiveness of the growing point to moisture, so that it grows in the 
direction in which tlie moisture lies. A Poplar has been known to send its 
roots under a hard l oad into a ditch on the opposite side, and many like 
illustrations are known which gave rise to the expression as if the plant 
were conscious of the presence of water in any particular direction. 
Sach's experiment of growing Beans within a trough made of gauze full 
of wet moss showed that as soon as the tips of the radicles protruded 
below, having grown vertically downwards while within the moss, the 
attraction for the moisture of the moss in the trough — which was 
inclined to the horizon — caused the tips to penetrate it again, (rravity 
