MOTILE ORGANS OF LEAVES. 689 



to an extent corresponding to the above numbers, but v/ithout undergoing transverse 

 division. As in all similar cases of curvatures due to grovfth— e.g. even in the case of 

 tendrils and, as we shall see later, of geotropically curved root-apices— the tissue of 

 the side which has become convex consists of large cells, containing abundance of 

 water and relatively little protoplasm^; that of the concave side, like very young 

 tissue, of small cells with little water and much protoplasm. 



That the motile organs of periodically motile compound leaves can execute 

 geotropic curvatures I first showed in 1865 in my ' Handbook,' employing the figure 

 here reproduced (Fig. 388) : it represents a young Kidney Bean which together with 

 its flower-pot was placed for about 4-6 hours in an inverted position, with the apex 

 downwards and excluded from light. The petioles had the directions indicated by 

 the arrows when the plant was inverted; but in consequence of geotropism the 

 motile organs P P^P^ became curved as represented in the figure, and thus the 

 petioles, which are not at all geotropic on their own account, assumed the positions 

 shown. Pfeffer showed subsequently that in these geotropic'curvatures of the motile 

 organs, it is not a corresponding growth of the down-turned upper sides which 

 occurs, but only a very considerable extension of the cells, combined with the taking 

 up of water, and later, when the plant is once more placed upright this is again 

 completely compensated after about 24 hours. It is not until the plant has remained in 

 the inverted position for several days that a permanent elongation by growth takes 

 place on the convex side. The motile organs therefore afford an opportunity of 

 confirming the theory of growth which I had previously established, in so far that, 

 according to this theory, any growth of the cell-walls is preceded by marked ex- 

 tension due to turgescence, and this, as we see, can be again annulled even though 

 it has attained a very considerable value, because the growth which takes place, in 

 consequence of the extension due to turgescence, only follows some time after- 

 wards. 



Passing now to a more detailed description of the processes which occur during 

 the downward curvature due to geotropism, I shall again select an object which, in 

 consequence of the geotropic stimulation, finally places its free end quite vertical. 

 This is very generally the case with the primary roots of the seedlings of Dicoty- 

 ledons, although other organs also, e. g. the first seed-leaf of many Monocotyledons, 

 such as the Kitchen Onion and the Date Palm, behave similarly, and again many 

 lateral roots, especially those springing from stems, enter the earth vertically. The 

 lateral roots which grow obliquely downwards, and other organs which behave 

 similarly, I shall for the time being leave out of consideration; we are only con- 

 cerned with rendering clear what happens in the case mentioned. 



In such experiments with roots not only is great precaution necessary, but also 

 the experience of years and an extensive knowledge of vegetable physiology, to 

 avoid falling into errors, as did Charles Darwin and his son Francis, who, on the 

 basis of experiments which were unskilfully made and improperly explained, came 

 to the conclusion, as wonderful as it was sensational, that the growing-point of the 

 root, like the brain of an animal, dominates the various movements in the root. 



It is not necessary here to enter in any way into a refutation of this view, since 

 this has been done by Detlefsen in the most forcible manner. It is true that, to 

 demonstrate the geotropic irritability of a root from which the growing-point has 



[3] vy 



