874 
MECHANICS OF GROWTH, 
{a) The most important result of Pfeffer's researches is doubtless the establishment 
of the fact that these movements depend not upon alternate expansion and contraction 
of the tissue, as was formerly thought, but upon modifications of growth. We must 
therefore distinguish between these movements and those of special organs which are 
no longer growing, and we may classify the former along with heliotropic and geotropic 
curvatures and with the movements of tendrils. It must not, however, be forgotten that 
all those external and internal conditions which increase or diminish the turgescence of 
the tissues must also accelerate or retard growth ; hence it follows that the same causes 
which modify the state of tension of a fully-developed organ may also modify the growth 
of organs which are still growing. If this takes place to a differing extent in the two 
sides of a bilateral organ, movements produced by growth will be exhibited. It might 
be suggested that all the movements of curvature treated of in this and in the fol- 
lowing chapter should be considered together. I quite agree with this suggestion as 
far as it goes, but an account contained in a text-book must possess clearness and 
precision, above all things, in the arrangement of the matter in hand. In the present 
incomplete state of our knowledge of the movements of curvature, these objects will be 
best attained if those movements which are results of growth be sharply separated from 
those which are independent of growth. 
{b) As regards Lea'ves the following m.ay be appended to the account previously given. 
In order to demonstrate that each upward and downward movement of the leaves of 
Chenop odium album is accompanied by an increase in length, Batalin fixed a straw seven 
or eight centimetres long as an indicator to the base of the lamina of a leaf attached to 
a stem which had ceased to grow ; the indicator projected laterally fromx the leaf and its 
movements were recorded by a tracing made by its free end upon a surface of sooted 
paper. It became apparent that the curves described by the point of the indicator cor- 
responding to each upward and downward movement did not coincide, but formed a 
zigzag line tending away from the stem. 
According to Pfeifer, the leaves of Impatiens, Chenopodium, Nicotiana^ and Wigandla 
exhibit, when in continuous darkness, a movement resembling that of the ordinary daily 
period, but this periodic movement does not continue for any length of time. The 
circumstance that the movement takes place, under these conditions, with the same 
intervals of time as in the ordinary period when the plant is exposed to the alterna- 
tion of day and night, opposes the assumption that the movement is due entirely to 
internal causes, that is, that there is any 'independent periodicity.' On the contrary, 
Pfeffer is inclined to assume that we have in this an instance of persistent effect whereby 
those movements are produced in total darkness which had been previously brought 
about by the daily alternation of light and darkness. These movements, produced by 
persistence of eflect, are accompanied by the growth of a particular side at a particular 
hour. It is not certain if, in addition to this, spontaneous nutations take place at shorter 
intervals of time. 
From Pfeffer's manuscript I take the following. When motile leaves are placed in 
the dark an acceleration of the growth of both sides is the result, just as greater 
turgescence and tension of the tissues is produced in the motile organs of leaves which 
have ceased to grow {Mimosa, Papilionaceae). 
A slight shortening of the side which is becoming concave may occur, as in the case 
of the geotropic curvature of the nodes of Grasses and of the curvature of stimulated 
tendrils. 
The property in virtue of which the leaves respond by an acceleration of growth 
to exposure to darkness is gained by previous exposure to light, but quite indepen- 
dently of assimilation. Leaves of Impatiens nolitangere make a distinct movement when 
replaced in darkness after an exposure of five minutes to light, and after an exposure of 
ten minutes the movement is well-marked. Growth is accelerated when the plant is 
replaced in darkness and takes place with greater rapidity than if the plant had remained 
continuously in the dark. The leaves of other plants {Sieges bee kia^ Chenopodium) require to 
