1 1 8 Bateson. — On the change of shape 
time thicker. He gave reasons for believing that this pheno- 
menon is essentially the same as the lengthening of ordinary 
pith in water, although in this latter case no narrowing in the 
transverse diameter was at that time known to occur. DeVries’ 
view that 4 the contraction of roots is a special form of cell- 
extension 5 is supported by the following experiments, in which 
as the pith becomes longer it, in many cases, diminishes 1 in 
transverse diameter, and thus supplies a resemblance between 
the two phenomena with which De Vries was not acquainted. 
Method. An oblong piece of turgescent pith of about 5 mm. 
in thickness 2 and 10-15 mm - in length, cut from the stem of 
a plant, was placed on its side in a flat-bottomed vessel and 
the changes in thickness taking place on the addition of water 
were noted. The measurements were made by means of a 
vertical micrometer-screw graduated to - oi mm. The screw 
carries a vertical needle at its lower extremity, which serves 
to make contact with the surface of a few drops of oil 
contained in a minute cup resting on the upper side of the 
pith. If the thickness of the pith varies, the cup of oil is 
necessarily raised or lowered by amounts easily read off by 
raising or lowering the screw until the needle-point once more 
dimples the surface of the oil 3 . 
The only difficulty connected with the method is that it is 
not easy to get pieces of pith symmetrically freed from 
external resisting tissues; unless this is carefully done the 
pith is liable to bend as it absorbs water. This source of 
error was foreseen, and all possible care was taken to over- 
come it. 
Helianthus annuus , the Sunflower. 
The pith having been placed in the flat-bottomed vessel 
1 This is not the case in all the species examined. 
2 Thickness is used in the ordinary sense of a measurement at right angles to 
the longitudinal axis. 
3 This method, or a modification of it, is a convenient one for many other 
purposes. For instance, if the oil-cup is fixed to the top of the micrometer-screw 
it is easy to measure the rate of growth of a plant. The string attached to the 
plant passes over a pulley in the usual way, and the weight bearing a needle- 
point makes contact with the oil-surface. 
