182 
moment that the annulus returns to its original position, a bubble 
of air makes its appearance in each of its cells. The sporangium 
when in glycerine rarely comes to rest so widely open as when dry. 
Now, first, what is the initial cause of the arching back of the 
annulus? Second, why does it return to its former position? 
Third, why does it finally assume the rectilinear position? The 
ordinary explanation is that the ring is elastic or hygroscopic. 
This is plainly unsatisfactory. It has been suggested by a late 
writer* that there is greater contraction of the thin external wall 
which thus pulls the annulus back. This explanation is disputed 
in a more recent paper by M. Leclerc du Sablon.t He argues 
that the external wall does not become tense as it would were it 
to contract more rapidly, but, on the contrary, sinks almost to 
the floor of the cell. Schinz offers still another solution, i. e., 
that the lignified walls are not homogeneous; that the inner lay- 
ers contain more water and consequently contract more upon 
drying than the external ones, and that to this unequal contrac- 
tion is due the initial movement of the annulus. M. du Sablon 
objects to this on the ground that if the annulus be separated 
from the sporangium and cut through lengthwise, each half 
ought to act precisely like the ring when intact. He claims to 
have made this section, but the half ring failed to contract. M. 
du Sablon’s own explanation is substantially as follows: “The 
dry air surrounding the sporangium causes the water contained 
in the cells to evaporate. The pressure of the surrounding medi- 
um is greater than the pressure inside the cells, and the delicate 
external wall is pushed in, which accounts for the concave appear- 
ance. The cells of the entire annulus being thus depressed, 
cause it to straighten and curve backward.” So far we can offer 
no criticism, although the explanation is purely hypothetical. He 
explains the return movement, however, in a very unsatisfactory 
manner. — He believes it to be caused by the sudden formation of 
bubbles of air in the row of cells constituting the annulus, which, 
‘according to his belief, increase the internal pressure sufficiently 
* ScuiNz.—Untersuchungen iiber den mechanismus des Aufspringens der Sporan- 
gien und Pollensiicke. Zurich, 1883. 
+ Recherches sur la dissemination des spores chez les Cryptogames vasculaires. 
Ann, des Sci. Nat., Sér. VII., Tom. II. 
