THE GAMETOPHYTE OF PSILOTUM. 
83 
Such rock does not weather and disintegrate readily, and such soil as is formed is extremely poor in 
fertility. It is evident that it must have mixed with it a large amount of organic material to enable a sapro- 
phyte such as Psilotum to develop. 
It will be noted that the amount of carbonic anhydride present is greater than the amount required to 
form carbonate with the magnesium and calcium oxides present ; the surplus is probably combined to form 
ferrous carbonate. 
While the solution of the ferrous carbonate as a sesquicarbonate, and its redeposition as hydrated iron 
oxide in the neighbourhood of streams described above, is a feasible chemical explanation, it is possible that 
the reactions are brought about or are accompanied by reactions set up by living organisms ; and it is note- 
worthy that where the deposits are being formed an enormous number of diatoms, which are well known to 
form for themselves siliceous skeletons, are present. 
.In this connection the suggestion of Sir T. H. Holland * is of interest. After discussing the constitution 
and formation of laterite, more especially as it occurs in India, he summarises his conclusions as follows : — 
(1) Laterite has generally been referred to as a ferruginous clay; but if the term clay is restricted to 
substances having a basis of hydrous silicate of alumina, this definition is incorrect. The alumina in laterite 
exists, as it does in bauxite, in the form of hydrous oxides. Kaolin must thus be removed finally from the 
list of weathering products ; it is formed generally, perhaps exclusively, by the action of subterranean vapours 
on aluminous silicates. 
(2) To account for the fact that an aluminous silicate undergoes a more complete disintegration under 
tropical conditions than under the deep-seated and presumably high temperature conditions of kaolinisation, 
the writer suggests that laterite is due to the agency of lowly organisms, possibly akin to the so-called 
nitrifying bacteria. With these there are probably forms akin to the bacteria which oxidise and fix ferrous 
compounds, and which, precipitating the silica in the colloid form, permit its removal by dilute alkaline 
solutions, simultaneously formed. This would account for the facts (a) that laterite is confined to the tropics, 
or at least is more conspicuously developed under tropical conditions ; ( b ) that although the laterite cover is 
100 feet or more in thickness, there is a sharp change from the soft-decomposition product to the absolutely 
fresh rock below ; (c) that though laterite can form at temperate altitudes, it is not observed in temperate 
latitudes, where, with a similar average annual temperature, there is a prolonged winter ; and ( d ) that laterite 
is a superficial product. 
2. The Spores and Germination. 
The spores of Psilotum are bean-shaped bodies measuring on the average 64'8 x 
32 ‘4 p. (Very rarely a spore nearly double this size may be found.) In the natural 
state they are quite separate from one another at the time of the opening of the 
ripe synangium, and form a very pale yellow glistening pile which is quickly dispersed 
as a small cloud by the wind. In the laboratory synangia that are nearly ripe will 
frequently open on keeping them for a day or two in a covered glass dish, and the 
spores, instead of being deposited in a small heap, will be all held together in a little 
yellow ball by a delicate meshwork. Such spores as are not absolutely mature 
germinate with difficulty. 
The mature spore has delicate reticulate epispore structures, and adheres very 
readily to even a smooth surface, such as glass. The dry. spore is a curved bean- 
shaped body with a narrow ridge joining the two ends of the curve — the regions on 
either side adjacent to the middle portion of the ridge being depressed. 
Along the ridge in the centre a median slit extends for about three-quarters of its 
* “On the Constitution, Origin, and Dehydration of Laterite,” Sir T. H. Holland, Geological Magazine, 1903, 
vol. x, p. 59. 
