Cytological Study on Induced Apospory in Ferns

Abstract

The study was carried out in order to bring about a better understanding of the life cycle of the pteridophyte by means of experimentally induced apospory. Inducing apospory was successful in Pteridium aquilinum var. latiusculum and Dryopteris erythrosora, but the former was exclusively used in the precise study because of the rapid growth of the gametophyte and the abundant induction of apospory.
The leaf or the root detached from the sporophyte produced the aposporous prothallium, but those attached to the sporophyte did not, irrespectively of the environmental conditions so far as the present study was concerned. Also the surgically damaged apex or undeveloped leaf attached to the sporophyte did not produce the prothallium. The ability of inducing apospory was limited to the early leaves or roots of the young sporophyte. The prothallial outgrowth occurred irrespectively of the leaf age and of the existence of the dividing cell. It originated from any part of the leaf without any correlation to the existing sporophytic structures, such as the leaf apex, the vascular bundle, etc. There was no polarity for the appearance of the outgrowth. Every sporophytic cell in the epidermis, the mesophyll and the cortex could produce the gametophytic cell except the guard cell and the vascular element. The outgrowth originated from one to several sporophytic cells. The first indication of the occurrence of the aposporous outgrowth was that many, large chloroplasts appeared in the sporophytic cells from which the outgrowth originated. But there were the leaves whose cells remained alive and green even after producing the aposporous outgrowth. The outgrowth on the detached root originated from the epidermis cells at or rather near the growing point. The case was essentially the same with that of the leaf.
The outgrowth was morphologically and functionally a true gametophyte and bore the sex organs by which the tetraploid sporophyte was sexually produced. Neither the sporophytic nor the intermediate outgrowth occurred on the detached sporophytic organ.
Also the probably tetraploid prothallium was aposporously induced on the detached tetraploid leaf.
The size of some characters of the aposporous diploid prothallium, such as the nucleus, the rhizoid, the spermatozoid and the antheridium, was compared with that of the normal haploid prothallium. The former was generally larger than the latter.
The tetraploid sporophyte exhibited many deviations from the normal morphology and physiology of the diploid sporophyte. They were the irregularly shaped leaf, the thick leaf, the uneven leaf surface, the dark green, the hairiness, the dwarfness, the anomalous stomata and the cold-hardiness. Each tetraploid plant may exhibit a different combination of them. There were apparently normally shaped plants too. The dimensions of the stoma and the epidermis cell on the tetraploid leaf were compared with those of the diploid leaf. The former was larger than the latter.
The factors responsible for inducing apospory were also discussed.
The writer wishes to express his sincere thanks to Profs. Drs. Masao Kumazawa and Ichitaro Harada of Nagoya University for their kind guidance and encouragement throughout this study, and in addition, to the latter for the correction of this manuscript. His thanks are also due to Prof. Dr. Motozi Tagawa of Kyoto University for identifying the ferns very kindly.