412 
PHYSIOLOGY. 
BOOK II. 
naturally assume a direction towards the heavens ; so that 
light thus becomes an aid to gravitation. It might even be 
believed that light alone was a sufficient cause of the perpen- 
dicular position of the stems of vegetables, if experience did 
not prove the contrary. Dutrochet laid horizontally on the 
ground, in a dry and dark place, the stems of Allium Cepa 
and Allium Porrum, taken up with their bulbs. These plants, 
although taken out of the ground, continued to live for a long 
time ; their stems became curved, and their upper end took a 
direction towards the heavens. This happened in about ten 
days ; but, being repeated in the open air, three days were 
sufficient to produce the direction. In the first experiment, 
light being wholly excluded, gravitation only could have 
operated in giving the stem a perpendicular direction ; that 
power being the only one which is known to act in a direction 
perpendicular to the horizon. Modifications of this experi- 
ment were instituted, to be certain that humidity had no 
effect, and the same result w^as obtained. In the prosecu- 
tion of these investigations, it also appeared that it was 
not merely the summit of the stem which had a tendency to a 
perpendicular direction, but that all the movable parts of 
the plant possessed a similar disposition, provided they were 
coloured. 
Stems are sometimes directed towards the earth, in which 
they attempt to bury themselves like roots ; a phenomenon 
worthy of the greatest attention, not only on its own account, 
but for the sake of the circumstances connected with it. Many 
vegetables, besides their above-ground stems, have also subter- 
ranean stems ; these creep horizontally in the interior of the 
earth, without manifesting any tendency towards the sky; 
they are white, like roots, of which they assume the course 
and the station. Sometimes, however, they are pink, as in 
Sparganium erectum ; in such cases it is the epidermis that is 
coloured, and not the subjacent parenchyma: but, when- 
ever the point ♦‘of their stems approaches the surface of the 
soil, it becomes green, and, from that moment, they acquire 
an upward tendency. Is it hence to be inferred, that there is 
some secret connection between the colours of the parts of 
vegetables and the directions they assume ? 
