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III. — The G-ametophyte of Psilotum. By G. P. Darnell-Smith, B.Sc., F.I.C. 
Communicated by Professor Anstruther Lawson, D.Sc. (With Two Plates.) 
(MS. received September 4, 1916. Read December 4, 1916. Issued separately May 23, 1917.) 
1. Introduction. 
Considerable interest attaches to the unravelling of the complete life-history of 
the Psilotacese, since they are apparently the nearest living representatives of the 
Sphenophy llales. 
The recent description of the gametophyte of Tmesipteris by Prof. Lawson, whose 
figures 1 had the privilege of seeing before they went to press, and the discovery of 
the gametophyte of Psilotum, which is the subject of the present paper, form a first 
contribution to the solution of this problem. It is of some interest to note that 
there remains now no Pteridophyte the gametophyte of which has not been described. 
In the early part of this research I was fortunate in finding a locality near at 
hand in which Psilotum flourished, and therefore I had at my disposal abundance of 
spores and later numerous prothalli. 
A general description of the distribution and habitat of Psilotum as far as its 
occurrence in the Hawkesbury sandstone around Sydney is concerned will serve as 
a fitting introduction to the more detailed consideration of the gametophyte. 
Psilotum is most usually found growing in the earth and vegetable debris that 
collect in deep horizontal fissures which occasionally occur in almost vertical sand- 
stone rocks. These rocks are massive, and'the fissures vary in width from a few inches 
to a fraction of an inch ; if they are very wide and contain much earth, Psilotum is 
not generally to be observed in them, and it seldom occurs in vertical or almost 
vertical fissures. The earth in the horizontal fissures generally contains a very large 
admixture of coarse sand, a considerable amount of decayed vegetable matter, and 
abundant remains of Julus oniscus and various Coleoptera. As a rule there is very 
little clay mixed with the earth ; and if this earth be shaken up with water, the sand 
and earth sink almost immediately on standing, and the supernatant liquid is com- 
paratively clear and free from colloidal particles. 
Rocks in the fissures of which Psilotum is found to be growing are noticed 
frequently to be somewhat basin-shaped above ; in this cavity water collects, and in 
time a shallow bog with a sandstone bottom is found. From this bog water 
gradually percolates through the sandstone rocks, keeping the fissures below just 
moist. In fact, while the aerial parts of Psilotum are admirably adapted to with- 
stand extremes of heat and- dryness and markedly xerophytic in character, the 
rhizome appears to require to be constantly surrounded by a damp atmosphere. 
TRANS. ROY. SOC. EDIN., YOL. LII, PART I (NO. 3). 12 
