II GERANIACE^ 133 



densities. There are usually two systems of parallel 

 lamellae, and they may be inclined to the axis of the 

 cell at almost any angle. Very frequently the two 

 systems wind spirally round the axis in opposite 

 directions. Now, according to Hofmeister,' when the 

 tissue of the cell - wall expands during imbibition it 

 is chiefly due to the swelling of the less dense 

 striae ; and we have seen that these striae are spirally 

 arranged, therefore it is probable that the imbibition of 

 water will result in spiral tension ; and spiral tension 

 will result in torsion — just as when a string is fastened 

 to one end of a rod, and is coiled spirally round it, and 

 the free end is pulled, the rod will tend to rotate on its 

 axis.^ Francis Darwin found that individual cells rolled 

 themselves up. If this explanation be correct, it would 

 seem to follow that the rods of Geranium do not twist, 

 because the striae are transverse and not inclined. 

 Hildebrand, on the other hand, accounted for the action 

 of the awn by the difference between the contraction of 

 the woody fibres and that of the comparatively soft 

 parenchyma, while Leclerc du Sablon accounts for it by 

 the fact that the outer cells of the rod are more com- 

 pletely lignified, and therefore contract more than the 

 inner ones. The spiral winding he explains by -the 

 fact that the woody bands are curved with the con- 

 cavity outwards. In Geranium, when the rods are not 

 spiral, the bands are not curved. 



The amount of the spiral twisting in the awn 

 depends upon the degree of moistness ; and the seed may 

 thus be made into a very delicate hygrometer, for if it be 

 fixed in an upright position the awn twists or untwists 

 according to the degree of moisture, and its extremity 

 thus may be so arranged as to move like a needle on 

 a register. It is also affected by heat. Now if the 

 awn were fixed instead of the seed, it is obvious that, 

 during the process of untwisting, the seed itself would 



' Lehre v. d. PflanzenzelU (1867). 



^ Francis Darwin, "On the Mechanism by which certain Seeds bury them- 

 selves in the Ground," Trans. Linn. Soc. ser. 2, Bot. i.. 



