REMARKS ON THE GROWTH OF PLANTS. 
327 
any topical application was, the sooner a wound would be healed 
thereby. 
The tendency of the vital membrane, or roots, towards darkness or 
any solid opaque body, is amusingly exemplified by the roots of orchi- 
deous plants suspended in a damp stove; for though they may be pro¬ 
duced towards the lightest side of the house, they soon trend round, 
and lengthen towards the back wall. A very expert cultivator of this 
tribe of plants, attributed the flexure of the roots to the attraction of 
the solid brick-work, and not to their inclination for either greater 
damp or darkness, nor yet to their aversion to light. The inclination 
or flexure of the stems and roots of plants seem to be opposed to each 
other, as if actuated by a contrary polarity. While the latter retire 
from, the former seek and turn to, the light. These movements are 
curious, and not easily accounted for. Every one is aware of how 
instinctively all plants present their growing surfaces to the strongest 
light. If healthy plants be placed in a dark room, and a single ray of 
light be admitted at the smallest aperture, soon all the points of the 
shoots and discs of the leaves are turned towards the stream of light. 
This is a proof that the plants are affected in some way by light, and 
either by attraction or irritation. 
Professor Rennie, we believe in one of his scientific alphabets, 
accounts for this phenomenon by comparing it with the contraction of 
damp paper when held before a fire, or to the flexure of a board laid on 
moist ground. In both cases the moistest side swells, or rather remains 
swollen, and consequently occupies more space than the dried side, 
which is contracted. The effects in these cases are quite natural, and 
easily understood; but whether applicable to the somewhat similar 
motions of plants, is not so evident, unless we could suppose that there 
is also a current of drying air ejected along with the light. But this 
we cannot readily conceive, when we look at the shoot of a potato at 
the further end of a dark vault (where no current of dry air can reach), 
presenting its top and leaves to the faint glimmer of light from the key¬ 
hole of the vault-door. In such a confined place as this, and where 
there can be little or no circulation of air, and very little difference 
in the state of the air as to humidity, we are led to conjecture that 
light has some peculiar effect upon plants,' independent entirely of 
moisture. 
The tendencies of roots towards their aqueous or gaseous food may be 
accounted for by supposing that an imperceptible vapour and effluvia 
are ever escaping from the sources, and these attracting the extreme 
spongioles, lead them onward to their food. As the roots, as well as the 
other growing members of a plant, are merely passive organs, they can 
