Observations on the male nucleus during fertilization in the fern Pteridium aquilinum

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Abstract
Fertilization of eggs of Pteridium under controlled conditions has revealed that archegonia containing viable eggs require more than 30 min flooding before opening. Syngamy appears to take place by an engulfing action, and no evidence was found of a ‘membrane-fusion’ kind of conjugation. Although the gametic nucleus lacks a recognizable envelope, as soon as it comes into contact with the female cytoplasm an envelope is reconstituted from an opaque layer at the surface of its chromatin. This membrane also undergoes growth, the outer layer extending into the cytoplasm and surrounding spermatozoid mitochondria. The male envelope remains distinct from the female, and lacks evaginations and well defined pores. The male nucleus lies in irregular loops within a funnel, filled with egg cytoplasm, formed by the microtubular ribbon of the gamete. The mouth of the funnel is bordered by the multilayered structure. Karyogamy is initiated at the anterior end of the male nucleus, the orifice connecting the 2 nuclei widening from about 0·7 to 2 μm in diameter during fusion. The contents of the male nucleus flow through the mouth of the funnel into the female, the chromatin relaxing locally, but retaining a rod-like form. The envelope of the male nucleus is probably resorbed at the site of junction. There is no evidence that it contributes significantly to the envelope of the zygotic nucleus.