. The origin of a land flora, a theory based upon the facts of alternation. Plant morphology. 56 ALTERNATING GENERATIONS but Dr. Lang's drawing shows how, nevertheless, the prothalli in turn hasten to a fresh apogamy (Fig. 38), thus the two transitions may be repeated at near intervals of time. The same was seen to be the case in. Trichomanes alatum, in which apospory and apogamy were found to succeed one another,1 and it has recently been proved to hold also for Athyrium fiiix foemina, var. clarissima, Jones, and other Lastly, Goebel has showns that when the seedling leaves of certain
. The origin of a land flora, a theory based upon the facts of alternation. Plant morphology. 56 ALTERNATING GENERATIONS but Dr. Lang's drawing shows how, nevertheless, the prothalli in turn hasten to a fresh apogamy (Fig. 38), thus the two transitions may be repeated at near intervals of time. The same was seen to be the case in. Trichomanes alatum, in which apospory and apogamy were found to succeed one another,1 and it has recently been proved to hold also for Athyrium fiiix foemina, var. clarissima, Jones, and other Lastly, Goebel has showns that when the seedling leaves of certain ferns are removed and cultivated on a moist substratum, aposporous growths may be induced, which show sometimes the most intimate inter- mixture of characters of the sporophyte and the gametophyte. These developments appear to be similar in kind, though not in detail, to those described by Lang and others. It would doubtless be possible to erect upon such facts a superstructure of theory; but it is necessary to remember that by the abscision of a young part it is placed in an anomalous and extreme physiological position. It is improbable that such circumstances ever arose in the course of descent: and accordingly it must remain, a quite open question what bearing, if any, such observations have upon the evolutionary story. They demonstrate possibilities: but possibilities are not the equivalent of historical data. The rapid succession of the transitions thus actually seen in some Ferns from the ^sporophyte to the gametophyte, and the con- verse, give some colour to the suggestion made by Goebel, that the sporophytic buds he found in the deep-water specimens of Isoetes are to be viewed as extreme cases of the telescoping of the alternate This state of affairs is very nearly matched by certain Adiantums observed by Lang, in which numerous sporophytic buds were produced from the sorus. Examination showed that they sprang in certain cases from the sporangia themselve
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