414 
PHYSIOLOGY. 
BOOK II. 
whicli are produced by these shoots are entirely colourless, 
like roots ; and, instead of taking a direction towards the sky, 
as coloured stems would do, they lead downwards, pointing 
towards the centre of the earth. In order to take this position, 
the young shoot forces its way through the substance of the 
petiole which covers it; thus overcoming a mechanical obstacle, 
in its tendency towards the earth. This subterranean stem 
next takes a horizontal course, and does not assume any 
tendency towards the sky until the points become green.” 
Dutrochet has also remarked a similar phenomenon in roots. 
It is well known that exposed stems of many plants produce 
roots : when green, they turn upwards, as in Pothos and Cac- 
tus phyllanthus ; when colourless, they point downwards. 
Hence it is to be inferred, that stems do not descend merely 
because they are stems, but because their parenchyma is 
coloured ; and that roots descend, not in their quality of roots, 
but because their parenchyma is colourless. It seems, how- 
ever, that although this law is uniform in its operation in all 
terrestrial plants, yet that a deviation, or apparent deviation, 
from it exists in the parasitic Mistletoe. The radicle of this 
plant, which is of a paler green than the other parts, instead 
of turning towards the light, avoids it with so much pertina- 
city that it is impossible to induce it to take such a direction ; 
so that it seems to be repelled by light. Dutrochet does not 
seem to be able to satisfy himself of the reason of this excep- 
tion : but it appears to be by no means difficult to account 
for. We have seen that, in the direction of its radicle, nature 
has enabled it to fulfil its functions as a parasitic plant, by the 
attraction of the body on which it is placed, rather than by 
the much more powerful attraction of the earth. In order 
to insure this particular tendency, without possessing which 
the existence of the Mistletoe would be put in hazard, its 
root has received, from the same all-powerful Hand, a dispo- 
sition so much greater than other plants to avoid light, and 
to bury itself in the obscurity of the interior of a tree, as 
to he sufficient to overcome the influence of its green colour- 
ing matter. 
The next direction of the parts of plants, which may be 
called special, is that of the upper surface of the leaves 
