In the young antheridia of Chara (Fig. 112), and in the not dissimilar case of the sporangium (or conidiophore) of Mucor, we easily recognise the hemispherical form of the septum which shuts off the large spherical cell from the cylindrical filament. Here, in the first phase of development, we should have to take into consideration the different pressures exerted by the single curvature of the cylinder and the double curvature of its spherical cap (p. 221); and we should find that the partition would have a somewhat low curvature, with a radius less than the diameter of the cylinder; which it would have exactly equalled but for the additional pressure inwards which it receives {304} from the curvature of the large surrounding sphere. But as the latter continues to grow, its curvature decreases, and so likewise does the inward pressure of its surface; and accordingly the little convex partition bulges out more and more.
In order to epitomise the foregoing facts let the annexed diagrams (Fig. 113) represent a system of three films, of which one is a partition-wall between the other two; and let the tensions at the three surfaces, or the tractions exercised upon a point at their meeting-place, be proportional to T, Tÿ£¢ãý and t. Let öÝ, öý, ö° be, as in the figure, the opposite angles. Then:
- (1) If T be equal to Tÿ£¢ãý, and t be relatively insignificant, the angles öÝ, öý will be of 90ô¯.
Fig. 113.
- (2) If T =ã₤Tÿ£¢ãý, but be a little greater than t, then t will exert an appreciable traction, and öÝ, öý will be more than 90ô¯, say, for instance, 100ô¯.
- (3) If T =ã₤Tÿ£¢ãý =ã₤t, then öÝ, öý, ö° will all equal 120ô¯.