. The origin of a land flora, a theory based upon the facts of alternation. Plant morphology. DICKSONIEAE 593 series of actively dividing cells. In sections perpendicular to the leaf- surface the young sorus appears as in Fig. 330 a, the cell marked (m) being one of the marginal series. A section of a similar sorus in a plane (x, x) appears as shown in Fig. 330 D (the chain of 10 cells superposed on the lip of the indusium), while at the lower level (y, y) jt is as shown between the indusial lips. It is thus seen that the receptacle is structurally like a normal leaf-margin, a conclusion again
. The origin of a land flora, a theory based upon the facts of alternation. Plant morphology. DICKSONIEAE 593 series of actively dividing cells. In sections perpendicular to the leaf- surface the young sorus appears as in Fig. 330 a, the cell marked (m) being one of the marginal series. A section of a similar sorus in a plane (x, x) appears as shown in Fig. 330 D (the chain of 10 cells superposed on the lip of the indusium), while at the lower level (y, y) jt is as shown between the indusial lips. It is thus seen that the receptacle is structurally like a normal leaf-margin, a conclusion again supported by Fig. 330 b, which shows a similar sorus traversed in a plane (z, z). Fig. 330. Dicksonia Schieciei, Baker. ^4=section through a young sorus perpendicular to the leaf-surface; i, i~indusium ; w*=cell of marginal series. B=section of sorus parallel to the leaf-surface as along a line i, z, in Fig. A, showing receptacle bearing sporangia s} s. C=3. similar section bearing older sporangia. Z>=transveise section of a young sorus showing the two lips of the indusium (ind), and receptacle between them, as along a plane.}', y, in Fig. A. A section of the receptacle as in plane x,x, in A, is superposed on the lower indusial lip. The central figure shows sporangial stalks cut transversely. A-D X 200. E, F, G, H sporangia of Dicksonia Menziesii from four different points of view. X50. indicated in Fig. 330 a. It is important to note the inequality in size of the two lips of the indusium. Here it is only slight, but in forms to be subsequently described an increasing inequality, both in area and in substance, makes its appearance. The formation of the sporangia has begun in the marginal cells, and it is seen that their appearance is almost simultaneous, a point better shown in Fig. 330 c, which repre- sents a slightly more advanced stage. These marginal sporangia are succeeded by others produced in a basipetal sequence, but the succession is not long continued, and in D. C
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