CERTAIN SEEDS BURY THEMSELVES IN THE GROUND. 161 
I have diseussed the mechanism of the bend at some length, because of its extremo 
importance in the burying-process; for if ће awn did not terminate above in a non- 
twisting horizontal portion, the rotation would not be transmitted to the point of the 
seed, neither could there be any increase of vertical pressure resulting from the straight- 
ening of the awn. 
Torsion.—An explanation will now be attempted of the power of torsion which the 
cells possess. 
It is a general property of hygroscopie tissue that the cells composing it are thick- 
walled. I have already pointed out the difference in this respect which exists between the 
hygroscopic and the non-hygroscopic tissue of the Stipa-awn. Hildebrand (* Schleuder- 
früchte ”) mentions the thick-walled cells composing the awn of Avena. We should expect 
therefore that the power of torsion would depend on the manner in which the cell-walls 
are thickened. 
Before discussing my view, it will be well, first of all, to exclude any cause resembling 
the unequal internal and external tensions which are supposed to account for the torsion 
of trees; for this only shifts the difficulty one step further off, and does not account for 
the constancy of direction of torsion. We may now examine the structure of the cells 
which make up the hygroscopic tissue. А body which swells on imbibing water will 
expand equally in all directions, if the molecular interstices in which the water lies are 
symmetrically arranged in all directions. Anda cell will have no tendency to twist if 
its cell-wall expands equally in all directions. Therefore, if we are to account for the 
torsion of a cell by the expansion or contraction of its cell-wall, we must examine the 
molecular structure of the wall with special reference to the distribution of the capacity 
for absorbing water. Тһе well-known researches of Nageli* are directed to this very 
point. 
He there shows that the cell-wall is composed of parallel lamell:e of alternate degrees 
of density and refractive indices. The first series of lamellz are seen in transverse sections 
of elongated woody cells, as concentrie shells, alternately light and dark, and fitting inside 
each other. This appearance is well known as stratification or ** Schichtung.” Тһе other 
systems are essentially of the same nature, but are not so well marked. They give rise 
to the appearances known as “ Streifung " or striation ; these are series of parallel lines, 
alternately light and dark, traversing the surface of the cell, and are in reality the edges of 
parallel lamelle of alternate densities. There are usually two systems of parallel lamelle ; 
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 striæ; and we have seen that these stris are 
spirally arranged ; therefore we are led to expect that the imbibition of water will 
result in some kind of 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. And since there are two 
systems of spiral striation, the tension due to one system must be stronger than that 
t Lehre v. d. Pflanzenzelle, (1867) p. 197. 
* Münchener Sitzungsb. 1864, May & July. 
: 272 
