The Development of the Ascocarp of Lachnea scutellata

Abstract

The mature ascocarp of Lachnea is disk-shaped. The hymenium forms the upper surface, while the rim and lower surface are covered by a thick-walled cortical layer. The center is composed of rather loosely interlacing hyphae. The ascogonium is the penultimate cell of a row of about nine. The ascogonium is early surrounded by vegetative hyphae, the outer of which form the first part of the cortex, while those around the ascogonium remain active and give rise on one side to more of the cortex and on the other to hyphae which will produce paraphyses. When a part of the cortex is once formed, the development of the hyphae composing that part ceases. The cells between the cortex and hymenium, however, remain active and add to the cortex and to the hyphae which produce paraphyses. The ascogenous hyphae are large and branch profusely. At the ends of these are formed typical hooks, consisting of binucleate penultimate and uninucleate ultimate and antepenultimate cells. The two nuclei of a penultimate cell may fuse to form the nucleus of an ascus; or they may divide and give rise to the four nuclei of another hook. The uninucleate ultimate cell usually grows down and fuses with the antepenultimate cell, after which the two nuclei may give rise to the nuclei of another hook, or they may fuse to form an ascus. When the hymenium is first formed, it is covered by the younger setae of the cortex, but as its diameter is increased and its level raised by the multiplication of the number of asci and paraphyses, it comes to be exposed. No fusion of nuclei was observed in either the ascogonium or ascogenous hyphae, except where two nuclei fuse to form the primary nucleus of an ascus. The nuclei of the ascogonium and ascogenous hyphae appear to be entirely similar except for size, and the same number of chromosomes, five, persists throughout their divisions. When the chromosomes are first formed, they are frequently grouped in a mass resembling a second nucleolus. The chromosomes become connected with a centrosome which was not apparent during the resting stage. This centrosome divides, and the two daughter centrosomes come to be situated at the poles of the spindle. At metaphase the five chromosomes divide, and at anaphase five pass to each pole. The daughter nuclei are usually organized at some distance from each other, but sometimes they are so close together that they resemble fusing nuclei. The first division in the ascus is heterotypic. Synizesis is produced by the contraction of a single spireme. After synizesis the spireme splits longitudinally. The two halves come together again, after which the spireme contracts considerably and segments into five elongated chromosomes. A centrosome makes its appearance on the nuclear membrane and becomes connected with the chromosomes by linin fibers in the nucleus. The centrosome divides and the daughter centrosomes come to be situated at the poles of the spindle. The chromosomes divide transversely. As they approach the poles they appear to split longitudinally. The second and third divisions in the ascus are similar to those in the ascogonium. The spore wall does not appear to be formed by the fusion of astral rays.