320 VEGETABLE PHYSIOLOGY 



So long as no growth takes place this line is horizontal, 

 but as the indicator is displaced downwards by the descent 

 of the small weight attached to the first cord, which is 

 attendant upon any elongation of the axis of the plant, the 

 line actually traced during growth is a spiral. The rate 

 of the drum's revolution being known, the amount of the 

 elongation of the axis per hour can easily be calculated. 

 The actual augmentation of the plant's axis is magnified 

 in the record, in a ratio dependent upon the ratio between 

 the radii of the large and small wheels r and x. 



For the sake of simplicity of description it has been 

 assumed, in what has already been said, that the turgidity of 

 the cells in the growing member is uniform. This, however, 

 is far from being the case. There is generally a certain 

 variation in this turgidity in the different parts of the 

 elongating member. The simplest case which we may 

 consider is one which shows a difference in structure on two 

 sides ; such a member is described as dorsiventral. The 

 two sides will often show a difference of degree of tur- 

 gidity and consequently of rate of growth. If we consider 

 a leaf of the common Fern, we find that in its young con- 

 dition it is closely rolled up, the upper or ventral surface 

 being quite concealed. As it gets older it gradually unfolds 

 and expands into the adult form. This is due to the fact 

 that in the young condition the turgidity and consequent 

 growth are greater on the dorsal side of the leaf, so that it 

 becomes rolled up as described. As it gets older the maxi- 

 mum turgidity and growth change to the upper side and so 

 it becomes unfolded or expanded. These two conditions 

 are generally described under the names of hyponasty and 

 epinasty respectively. 



These conditions are not confined to the leaves of ferns, 

 but may be detected in those of other plants, though to a 

 less conspicuous degree. It is in consequence of them that 

 the leaves of the bud always fold over the apex of the stem 

 from which they spring. The opening and closing of certain 

 flowers, such as the Crocus, depend upon similar variations. 



