820 VEGETABLE PHYSIOLOGY 
So long as no growth takes place this line is horizontal, 
but as the indicator is displaced downwards by the descent 
of the small weight attached to the first cord, which ig 
attendant upon any elongation of the axis of the plant, the 
line actually traced during growth is a spiral. The rate 
of the drum’s revolution being known, the amount of the 
elongation of the axis per hour can easily be calculated. 
The actual augmentation of the plant’s axis is magnified 
in the record, in a ratio dependent upon the ratio between 
the radii of the large and small wheels r and z. 
For the sake of simplicity of description it hag been 
assumed, in what has already been said, that the turgidity of 
the cells in the growing member is uniform. This, however, 
is far from being the case. There is generally a certain 
variation in this turgidity in the different parts of the 
elongating member. The simplest case which we may 
consider is one which shows a difference in structure on two 
sides; such a member is described as dorsiventral. The 
two sides will often show a difference of degree of tur- 
gidity and consequently of rate of growth. If we consider 
a leaf of the common Fern, we find that in its young con- 
dition it is closely rolled up, the upper or ventral surface 
being quite concealed. As it gets older it gradually unfolds 
and expands into the adult form. This is due to the fact 
that in the young condition the turgidity and consequent 
growth are greater on the dorsal side of the leaf, so that it 
becomes rolled up as described. As it gets older the maxi- 
mum turgidity and growth change to the upper side and so 
it becomes unfolded or expanded. These two conditions 
are generally described under the names of hyponasty and 
epinasty respectively. 
These conditions are not confined to the leaves of ferns, 
but may be detected in those of other plants, though to a 
less conspicuous degree. It is in consequence of them that 
the leaves of the bud always fold over the apex of the stem 
from which they spring. The opening and closing of certain 
flowers, such as the Crocus, depend upon similar variations. 
