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IV. — The Gametophyte Generation of the Psilotaceee. By A. Anstruther Lawson, 
D.Sc., Professor of Botany, University of Sydney. (With Five Plates.) 
(MS. received December 16, 1916. Read February 5, 1917. Issued separately June 13, 1917.) 
Introduction. 
Since the preliminary announcement of the discovery of the prothalli of Tmesip- 
teris and Psilobum communicated to the Royal Society of Edinburgh in June 1916, # 
a considerable amount of additional material has been found, and it seems therefore 
desirable to place on record a more detailed account of these interesting structures. 
The preliminary description, as published, was based upon but a few specimens, and 
these were not sufficient to reveal all of the vegetative and reproductive features, nor 
enough to justify any broad general conclusions as to the phylogenetic position of 
the Psilotacese. The discovery, however, has filled in an important and interesting 
gap in our knowledge ; for the Psilotacese are the very last of the known Pterido- 
phytes to surrender their gametophytes to the light of science. 
While the present paper gives a fairly complete account of the vegetative 
character of the prothallus and the development of the antheridium and archegonium, 
it does not include the embryo. I have advisedly left this for a future contribution, 
realising that the embryology is the most important phase in the life-history of these 
plants. The additions to my stock of material have added to the embryo stages 
already reported, but these are not yet sufficient to show a complete unbroken series. 
I have therefore considered it advisable to defer the account of the embryo, feeling 
confident that I will be able to fill in the missing gaps. I have learned how and 
where to obtain material, and it is therefore only a matter of patient searching until 
the series is complete. 
Tmesipteris tannensis, Bernh. 
The Vegetative Features of the Prothallus. 
It is a generally accepted belief that Tmesipteris tannensis always grows upon 
the trunks of tree ferns. This idea, however, is not in accordance with the facts 
and not in harmony with my own observations. The plant certainly grows more 
luxuriantly upon the trunks of Dicksonia, and frequently on Alsophila and Todea 
barbara ; but in certain localities and under certain conditions I have found it in 
nature growing plentifully in soil and in the crevices of rocks. It was the discovery 
of it growing freely in soil which rendered it possible to find the prothalli in quantity. 
I had searched laboriously through many tree-fern trunks, and after patiently working 
* Lawson, A. A., 1916. 
TRANS. ROY. SOC. EDIN., VOL. LII, PART I (NO. 4). 
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