B40 
PHYSIOLOGY. 
BOOK II. 
of the vegetable tissue ; and can by no means be compared 
to that of the petals of some plants, which arises from the 
presence of a white colouring matter. Light, which is the 
principal, but not sole, cause of the colour of stems and their 
organs, has no power of infusing colour into the roots, as may 
be easil}^ seen by roots growing in glasses of water; in spite of 
the influence of the light they constantly remain colourless ; 
and this does not depend upon immersion in water, because 
leaves developed in that medium are nevertheless green. 
Although roots have, in general, no tendency towards the 
light, yet such a disposition does become manifest, provided 
the terminal shoot of a root becomes slightly green, as occa- 
sionally happens. Having induced some seeds of Mirabilis 
jalapa to germinate in damp moss, I remarked that the young 
roots, when about as long as the finger, were terminated by a 
shoot of a slightly green colovir. Wishing to know whether 
these roots would turn towards the light, I placed them in a 
glass vessel filled with water, having a w^ooden cover pierced 
with holes to receive the roots and fix the seeds. I enveloped 
the vessel in black cloth, leaving only a narrow vertical slit, 
through which lio^ht could enter the interior. This slit was 
exposed to the rays of the sun ; and, a few hours after, I found 
that all my roots had hooked back their points towards the slit 
through which light was introduced. The same experiment 
was tried with colourless roots; but no alteration in their 
direction was produced. From this it appears evident that 
colour is one of the conditions that determine the directions of 
vegetables and their parts towards the light, and consequently 
towards the sky. This is so true, that colourless stems are 
known to assume the directions of roots. In the Sagittaria 
sagittifolia this is particularly obvious. Shoots are produced 
from the axillae of all the radical leaves which grow at the bot- 
tom of the water. These shoots have their points directed to- 
wards the sky, like those of all vegetables. The young stems, 
which 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 
