862 MOVEMENTS OF PLANTS.—HELIOTROPISM.—GEOTROPISM. 
some species of Oxalis. The positions, indeed, which leaves 
occupy when asleep are almost infinitely diversified : they may 
either point vertically upwards ( fig. 1173,8) or downwards ( fig. 
1174, B); or, in the case of leaflets, towards the apex or towards 
the base of the leaf, or in any intermediate position. .. . 
‘The nyctitropic movements of leaves, leaflets and petioles 
are effected in two different ways—firstly, by alternately in- . 
creased growth on their opposite sides, preceded by an increased 
turgescence of their cells (see page 782 and fig. 1140); and 
secondly, by means of a pulvinus or aggregate of small cells, 
generally destitute of chlorophyll, which become alternately more 
turgescent on nearly opposite sides, and this turgescence is not 
followed by growth except during the early age of the plant.’ 
b. Not periodical.—Such movements are exhibited in a num- 
ber of plants both in the leaves and in their reproductive organs. 
In the leaves they are well seen in certain species of Oxalis and 
Mimosa (fig. 373), in Dionxa muscipula (fig. 375), &e. In the 
reproductive organs they may be noticed in the curving inwards 
or outwards of the stamens of certain plants, such as those of 
Berberis vulgaris and other species, Parietaria judiaca, Helian- 
themum vulgare and other Cistacez ; also in the stigmas of the 
Lobeliacez, aud in the style of Goldfussia anisophylla, &e. All 
the above movements are produced by external agency, such as 
the action of insects, the agitation caused by the wind, &c. 
Other movements which fairly come under this heading, and 
which, like the nyctitropic movements, are by Darwin regarded 
as being due to modified circumnutation, are positive and negative 
heliotropism, positive and negative geotropism, Ke. 
Positive heliotropism is the growing towards the source of 
light. It has been long known that plants grown in comparative 
darkness increase in length more rapidly than those exposed to 
a stronger light--i.e. that light appears to have a retarding 
influence on growth—therefore, where a plant or part of a 
plant exhibits positive heliotropism, it is found that the part 
away from the light has attained a greater length than that 
towards it. 
Some few vegetable organs, as the stem of Ivy, and many 
roots, exhibit negative heliotropism, where, as they grow away 
from the light, the parts next the source of illumination grew 
most. 
Positive geotropism or gravitation is the term applied to the 
force which influences the direction of growth of most roots, 
especially of primary roots, which usually point directly down- 
wards to the centre of the earth. 
Negative geotropism, on the other hand, signifies the direction 
taken by most stems, trees, &c., being exactly opposite to that 
sought by the roots—i.e. upwards, or away from the centre of 
the earth. 
As the terms positive and negative heliotropism and of posi- 
