Opening and Closing of Tulip Flowers. 57 
As a matter of fact there is good evidence to shew that in the 
perianth leaves of the 1 ulip there exists, on the outer face of each, 
a tissue comparable to that in the Dionaea leaf, and that it is 
capable of readily undergoing changes in the condition of the 
turgescence of its constituent cells, and thus is able to bring about 
the movements under consideration. 
If a section, which must not be too thin, be cut down the 
median line of a petal taken from a fairly young flower, and then 
be placed in water, the uninjured cells become turgid. And as the 
section becomes more or less strongly curved, it is clear that the 
cells on the outer side of the curve have expanded more than those 
on the inner side. It will be observed that this curvature is always 
such as would effect the closing of a petal still attached to the 
plant. The curvature is most striking near the base of the petal, 
but it may also involve the more apical region. If the turgid 
section be now transferred to a rather strong plasmolysing solution, 
e.g.,3 — 4% KNO„ it will be seen, after a few moments, to straighten 
itself rapidly; that is a movement will be executed in the sense 
of causing the petal to open. These opening and closing 
movements can be repeated on the same preparation a large 
number of times. It appears then that there exists on the outer 
face of each perianth leaf, and more especially in the region where 
the petal is naturally curved, a layer of active cells which are able 
to alter their state of turgescence, or at any rate their size, more 
readily and effectively than the cells which form the more internal 
tissue layers, and that it is to the agency of these cells that the 
opening and closing of the flower is to be ascribed 
It still however remains to be proved that we are dealing with 
an irritable tissue, and not with a more proximate mechanical 
effect of the water or salt upon the cell walls. If a section which 
is lying in water, and hence exhibits a strong (closing) curvature be 
transferred to alcohol, it is killed and fixed in the curved condition. 
On placing it afterwards in either water or in the salt solution, it 
speedily becomes relaxed, and straightens out; thus shewing that 
its previously curved appearance was directly related to the life and 
activity of the protoplasm of its cells, and to the increased size of 
those on the outer face under circumstances which permitted the 
attainment of a maximum degree of turgescence. 
The anatomical relations of the tissues of the perianth leaves 
are well adapted to facilitate rapid changes of curvature. The 
intercellular spaces are specially obvious in the tangential direction, 
