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. 



(&) As regards Lewves the following may be appended to the account previously given. 

 In order to demonstrate that each upward and downward movement of the leaves of 

 Chenopodium 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 from 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 Pfeffer, the leaves of Impatiens, Chenopodium, Nicotiana, and Wigandia 

 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 vi^hich had been previously brought 

 about by the daily alternation of light and darkness. These movements, produced by 

 persistence of effect, 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, Papilionacese). 



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 r,emained 

 continuously in the dark. The leaves of other plants {Sieges bee kia, Chenopodium) require to 



