Morphology of Preissia quadrata

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Abstract
P. quadrata is the only species of a genus closely related to Marchantia. The air chambers, forming a single layer, arise by intercellular cleavage, the split starting below the epidermis. The structure of the thallus is described in detail. Branching is dichotomous at first, but after a receptacle has formed further growth takes place by an apical innovation. This arises beneath the receptacle and grows by means of a small cuneate apical cell. Both kinds of receptacles are stalked and represent a branch system. They are mostly unisexual and dioecious, rarely monoecious, but very commonly bisexual. A greater proportion of male receptacles appears during the early part of the season, and of female ones during the latter part. Bisexual receptacles are most common during the middle of the season. On them the production of antheridia always precedes the formation of archegonia. All plants contain the potentialities of both sexes. Receptacles are at first undifferentiated, and assume male or female form and structure depending on the kind and number of sex organs produced. The development of sex organs, described in detail, is typical. The ventral canal cell and egg are differentiated after 4 neck canal cells are formed; the number is not later increased. Several anomalous sex organs of intermediate type are described, giving evidence of a common origin phylogenetically for the antheridium and archegonium. The embryo develops with an octant stage; it lacks a functioning apical cell. The occurrence at time of fertilization of a centrosome with astral rays at opposite poles of the egg nucleus, previously reported, is confirmed. Haploid number of chromosomes is 9, one being very small.

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