CHAP. XII. 
DIRECTIONS. 
339 
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 was 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 moveable 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 cuticle 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 recent connection between the colours of the parts of 
vegetables and the directions they assume ? 
^' Generally," Dutrochet proceeds to remark, " stems are 
directed towards the light, which is in accordance with their 
colour, which is usually green ; while the roots have usually 
a tendency to avoid the light, which coincides with their want 
of colour. The colour of the roots is, in fact, nothing but that 
z 2 
