. The origin of a land flora, a theory based upon the facts of alternation. Plant morphology. JUNGERMANNIALES 265 the basis of the first segmentations, of the capsule from the seta. So far as segmentations offer a basis for argument, the general conclusion may be drawn that the seta and capsule are not always distinct ab initio. The developmental facts suggest that the fertile region may be held to be a residuum left by sterilisation, which has been basifugally progressive : the result of such sterilisation is the region of the seta as it is seen in these plants. We may regard as the most prim
. The origin of a land flora, a theory based upon the facts of alternation. Plant morphology. JUNGERMANNIALES 265 the basis of the first segmentations, of the capsule from the seta. So far as segmentations offer a basis for argument, the general conclusion may be drawn that the seta and capsule are not always distinct ab initio. The developmental facts suggest that the fertile region may be held to be a residuum left by sterilisation, which has been basifugally progressive : the result of such sterilisation is the region of the seta as it is seen in these plants. We may regard as the most primi- tive case that in which all the tiers of cells of the embryo form the archesporium : those cases in which the differentiation of the archesporium is deferred in the individual life may be held to be relatively less primitive. The four cells of the uppermost tier, which thus as a rule form the capsule in the Jungermanniales, divide first transversely to form four terminal cover- cells : the four larger cells below again segment to form four inner cells and eight to twelve peripheral cells. This is the same segmentation as occurs also in the lower tiers: the similarity is in accordance with the view above expressed as to the origin of the seta by basifugal sterilisation, and supports the conclusion that seta and capsule had a common origin. However interesting such questions may be, they are more or less speculative. A much greater interest, proportional to the greater cogency of the facts, attaches to the various modes of development of the capsule itself in the Jungermanniales. It has been seen that the inner cells above described constitute the arche- sporium. In many of the Akrogynous Jungermanniaceae the cells, after repeated divisions, undergo a differen- tiation as in the Marchantiaceae, into spore-mother-cells and sterile elaters : these are associated in various ways, and the case of Porella will serve as an average example (Fig. 126). lire argument from differentiati
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