Morphology of Preissia quadrata

Abstract

1. Preissia comprises a single species, P. quadrata, closely related to Marchantia. 2. The thallus is dichotomous when young, later forming apical innovations. It lacks a distinct midrib. Both smooth and pegged rhizoids and appendaged scales are borne below. 3. There is a single layer of air chambers with green filaments; their origin is schizogenous. The epidermal cells are thin walled and contain few chloroplasts. All air pores are barrel-shaped. 4. The colorless ventral cells are elongated and have thickened pitted walls. Scattered sclerotic cells occur in the ventral region. 5. Growth is by means of a single cuneate apical cell. After a receptacle has arisen, another apical cell forms an apical innovation which continues growth of the thallus. 6. The receptacles are mostly unisexual and dioecious, rarely monoecious, but very commonly bisexual. Both male and female receptacles are stalked, and both represent a branch system. Elongation of the female receptacle stalk is delayed. 7. Both receptacles have four growing points as a rule, the male showing greater variability in this respect than the female. The female receptacle has four inconspicuous lobes and four prominent ridges, the latter being incipient rays. Ordinarily about four or five archegonia are formed in each of the four groups. 8. A greater proportion of antheridial receptacles appears during the early part of the growing season, and of archegonial ones during the latter part. The greatest proportion of bisexual receptacles occurs during the middle of the season. 9. On bisexual receptacles the production of antheridia always precedes the formation of archegonia. 10. All of the thalli contain the potentialities of either sex. 11. The general form of a receptacle is determined by the kind and number of sex organs it produces. 12. The antheridia develop like those of the other Marchantiales. The formation of periclinal walls delimiting the spermatogenous cells involves the three uppermost tiers of cells of the young antheridium. 13. Early development of the archegonium is typical. The ventral canal cell and egg are differentiated after four neck canal cells are formed; this number is not later increased. 14. Several anomalous sex organs are described, giving evidence as to a common origin phylogentically for the antheridium and archegonium. 15. At the time of fertilization, at least in some cases, there is a centrosome with astral rays at opposite poles of the egg nucleus. The condition of the nucleus and cytoplasm at this time is described. 16. The haploid number of chromosomes is nine, one being very small. 17. The embryo develops with the octant stage characteristic of certain related genera. A functioning apical cell does not occur in the embryo of the Marchantiales.