We have innumerable cases, near the tip of a growing filament, where in like manner the partition-wall which cuts off the terminal {303} cell constitutes a spherical lens-shaped surface, set normally to the adjacent walls. At the tips of the branches of many Florideae, for instance, we find such a lenticular partition. In _Dictyota dichotoma_, as figured by Reinke, we have a succession of such partitions; and, by the way, in such cases as these, where the tissues are very transparent, we have often in optical section a puzzling confusion of lines; one being the optical section of the curved partition-wall, the other being the straight linear projection of its outer edge to which we have already referred. In the conical terminal cell of Chara, we have the same lens-shaped curve, but a little lower down, where the sides of the shoot are approximately parallel, we have flat transverse partitions, at the edges of which, however, we recognise a convexity of the outer cell-wall and a definite angle of contact, equal on the two sides of the partition. [Illustration: Fig. 110. Cells of _Dictyota_. (After Reinke.)] [Illustration: Fig. 111. Terminal and other cells of _Chara_.] [Illustration: Fig. 112. Young antheridium of _Chara_.] 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_, _T′_ and _t_. Let α, β, γ be, as in the figure, the opposite angles. Then: (1) If _T_ be equal to _T′_, and _t_ be relatively insignificant, the angles α, β will be of 90°. [Illustration: Fig. 113.] (2) If _T_ = _T′_, but be a little greater than _t_, then _t_ will exert an appreciable traction, and α, β will be more than 90°, say, for instance, 100°. (3) If _T_ = _T′_ = _t_, then α, β, γ will all equal 120°.