Fig. 104. Developing asci of Physcia ciliaris DC. × 800 (after Baur).
F. Bachmann[671] was able to make important cytological observations in her study of Collema pulposum. As regards the vegetative and ascogonial nuclei, five or perhaps six chromosomes appeared on the spindle when the nucleus divided. In the asci, the usual paired nuclei were present in the early stages and did not fuse until the ascus had elongated considerably. After fusion the definitive nucleus enlarged with the growth of the ascus and did not divide until the ascus had attained full size. The nucleolus was large, and usually excentric, and there were at first a number of chromatin masses on an irregular spirem. In synapsis the spirem was drawn into a compact mass, but after synapsis, “the chromatin is again in the form of a knotty spirem.” In late prophases the chromosomes, small ovoid bodies, were scattered on the spindle; later they were aggregated in the centre, and, in the early metaphase, about twelve were counted now split longitudinally. There were thus twice as many chromosomes in the first division in the ascus as in nuclear divisions of the vegetative hyphae. F. Bachmann failed to see the second division; there were at least five chromosomes in the third division.
Considerable importance is given to the number of the chromosomes in the successive divisions in the ascus since they are considered to be proof of a previous double fusion—in the ascogonium and again in the ascus—necessitating, therefore, a double reduction division to arrive at the gametophytic or vegetative number of five or six chromosomes in the third division in the ascus. There have been too few observations to draw any general conclusions.
c. Development of Spores. The spore wall begins to form, as in Ascomycetes, at the apex of the nucleus with the curving over of the astral threads, the nucleus at that stage presenting the figure of a flask the neck of which is occupied by the centrosome. The final spore-nucleus, as observed by Maire, divides once again in Anaptychia and division is followed by the formation of a median septum, the mature spore being two-celled. In Peltigera the spore is at first ovoid, but both nucleus and spore gradually elongate. The fully formed spore is narrowly fusiform and by repeated nuclear division and subsequent cross-septation it becomes 4- or even 5-6-celled.
The spores of lichens are wholly fungoid, and, in many cases, form a parallel series with the spores of the Ascomycetes. Markings of the epispore, such as reticulations, spines, etc., are rarely present (Solorina spongiosa), though thickening of the wall occurs in many species (Pertusariae, etc.), a peculiarity which was first pointed out by Mohl[672] who contrasted the spore walls with the delicate membranes of other lichen cells. Some spores, described as “halonate,” have an outer gelatinous covering which probably prevents the spore from drying up and thus prolongs the period of possible germination. Both asci and spores are, as a rule, more sparingly produced than in fungi; in many instances some or all of the spores in the ascus are imperfectly formed, and the full complement is frequently lacking, possibly owing to some occurrence of adverse conditions during the long slow development of the apothecium. In the larger number of genera and species the spores are small bodies, but in some, as for instance in the Pertusariae and in some Pyrenocarpeae, they exceed in size all known fungus spores. In Varicellaria microsticta, a rare crustaceous lichen of high mountains, the solitary 1-septate spore measures up to 350 µ, in length and 115 µ in width. Most spores contain reserve material in the form of fat, etc., many are dark-coloured; Zukal[673] has suggested that the colour may be protective.
Their ejection from the ascus at maturity is caused by the twofold pressure of the paraphyses and the marginal hyphae on the addition of moisture. The spores may be shot up at least 1 cm. from the disc[674].
d. Spore Germination. Meyer[675] was the first who cultivated lichen spores and the dendritic formation which he obtained by growing them on a smooth surface was undoubtedly the prothallus (or hypothallus) of the lichen. Actual germination was however not observed till Holle[676] in 1846 watched and figured the process as it occurs in Physcia ciliaris.
Spores divided by transverse septa into two or more cells, as well as those that have become “muriform” by transverse and longitudinal septation, may germinate from each cell.