. The structure and development of mosses and ferns (Archegoniatae). remain single-layered, and undergo but littlefurther modification beyond an increase in size. From thebase of the young leaves simple hairs develop, but remain smalland inconspicuous. The inner of the three first formed cellsof the segment, by further division and grow^th in all direc-tions, produces the axis of the plant. This in cross or longi-tudinal section shows almost perfectly uniform tissue. Nodistinct epidermis, or central strand, like that found in mostMosses, can be seen. I04 MOSSES AND FERNS chap; The branching is
. The structure and development of mosses and ferns (Archegoniatae). remain single-layered, and undergo but littlefurther modification beyond an increase in size. From thebase of the young leaves simple hairs develop, but remain smalland inconspicuous. The inner of the three first formed cellsof the segment, by further division and grow^th in all direc-tions, produces the axis of the plant. This in cross or longi-tudinal section shows almost perfectly uniform tissue. Nodistinct epidermis, or central strand, like that found in mostMosses, can be seen. I04 MOSSES AND FERNS chap; The branching is monopodial and the branch representsthe ventral lobe of a leaf. After the first division by whichthe two lobes of the leaf are separated, only the dorsal onedevelops into the lamina of the leaf, which is thus in the seg-ment from which a branch is to form, only one-lobed. In theventral cell three walls arise (Fig. 51), intersecting so as tocut out a pyramidal cell of the same form as the apical cell ofthe main axis, and the cell so formed at once begins to divide. y. Fig. 51.—Diagram showing the ordinary method of branching in the acrogynous Jun-germanniaceae (after Leitgeb). D, Dorsal; V, ventral side of stem; X X, apicalcells of the branches. The segments are numbered. in the same way, and forms a lateral axis of precisely the samestructure as the main one. The genus Physiotium differs from all other known Acrog-ynse in having a two-sided apical cell, instead of the typicaltetrahedral one—(Goebel (21), p. 287). The Sex-organs The plants in Porella are strictly dioecious and the two sexesare at once recognisable. The males are smaller, and bearspecial lateral branches which project nearly at right anglesfrom the main axis, and whose closely imbricated light green in THE JUNGERMANNIALES 105 leavei. make them conspicuous. At the base of each of theleaves is a long-stalked antheridium, large enough to be readilyseen with the naked eye. The development of the antheridium may
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