INTRODUCTION. 
«79 
condition, of curving geotropically and heliotropically. I observed both these sets 
of phenomena, for example, in the motile organs of Phaseolus, and in the irritable 
filaments of the Cynareae. These heliotropic and geotropic curvatures are necessarily 
accompanied by growth. To meet this it may be pointed out, (i) that the periodic 
movements and the movements due to irritation which we are now considering are 
not dependent upon growth but upon an alternate contraction and dilatation of the 
cells; (2) that there are other organs which are also capable of renewed growth 
under abnormal conditions after their growth under normal conditions has ceased. 
We found this to occur in the nodes of Grasses. When these have ceased to grow 
in the erect position, they exhibit sharp geotropic curvatures due to a vigorous 
growth of the lower surface when they are placed horizontally. The periodically 
motile and irritable organs evidently possess the same property. They are not only 
irritable and periodically motile after their growth under normal conditions has 
ceased, but they are capable of renewed growth under abnormal conditions. The 
conditions are abnormal when light falls upon these organs from one side only, or 
when they are placed in an unusual position which is more or less nearly horizontal ; 
then heliotropism and geotropism are brought into play. 
Now that sufficient stress has been laid upon the distinction between organs 
which are motile while growing and organs which are motile after they have ceased 
to grow, the points of resemblance of the two may be considered. The first of 
these is the fact that for each kind of movement which is exhibited by the special 
motile organs, a corresponding kind may be observed in growing organs. Thus, 
the spontaneous periodic movements of the former correspond to the spontaneous 
nutations of growing stems, leaves, tendrils, and flowers. The variations of tempe- 
rature and of the intensity of light which act as stimuli upon the former, by increas- 
ing or diminishing the turgescence of the tissues, also affect many growing leaves 
and cause flowers to open or shut, by accelerating the growth of one side or the 
other. The motile organs of Mimoseae, OxaUdese, and Cynareae, which are sensitive 
to contact, correspond to the growing tendrils and roots which are sensitive to 
pressure. 
This comparison tends to show that the causes which induce contraction and 
dilatation of the cells of motile organs by modifying their turgidity, may also retard 
or accelerate the growth of growing organs. The deeply-seated connection be- 
tween these phenomena will become evident if what was said in Sect. 14 as to the 
causes of growth be compared with the following account of the causes of the 
periodic movements and of those due to stimulation. According to my theory, the 
hydrostatic pressure which the cell-sap exercises, in consequence of its increase in 
quantity by endosmosis, upon the extensible cell-wall, is an essential condition of the 
growth of the cell ; fresh solid matter is deposited between the micellae of the 
stretched cell-wall, it therefore grows, and thus grow^th is a perpetual over-stepping 
of the limit of elasticity of the stretched cell-wall. Everything which increases the 
turgescence of the cell promotes its growth, everything which diminishes the tur- 
gescence is prejudicial to its growth. If these effects are produced in different 
degrees upon the two sides of a growing organ, corresponding curvatures will be 
produced. If these effects are produced in a mass of tissue the cell-walls of which 
have ceased to grow but are very extensible and very perfectly elastic, an increase of 
