SHOWING STRUCTURE, FROM THE RHYNIE CHERT BED, ABERDEENSHIRE. 895 
When we take into consideration the composition of the water of siliceous 
springs at the present day, it further seems quite possible that plants like Rhynia 
may have grown in a substratum saturated with the siliceous solution. In the 
light of what is known as to the vegetation found around siliceous springs at the 
present day, the xerophytic construction of the Rhynie plants may be of signifi- 
cance. In the case of the vegetation around fumaroles in Java * the plants are 
xerophytic, rooted in the hot acid soil, and with their foliage exposed to hot 
sulphurous vapours. The soil consists of a white siliceous clay. The clay under- 
lying the Rhynie deposit might possibly be regarded as of this nature, while 
corresponding in its effects on the vegetable remains above to the clay beneath 
recent peats. 
The possibility that the water saturating the Rhynie plant-remains might have 
been siliceous throughout their accumulation may further explain a most striking 
difference between the preservation of this bed, considered as a whole, from what 
is found in recent peats. In the case of the latter the best preservation is 
met with in the upper region, decay and compression being marked in the lower 
part of the bed. In the Rhynie deposit, on the other hand, the preservation is 
as good in the lowest beds of the eisdit-foot section as it is above. Further, the 
plant-remains at the base of the deposit are not compressed, and are often loosely 
placed in the matrix. It was suggested above that the water around the growing 
plants might have been siliceous, and that the decay going on in the accumulating 
remains might have determined the separation of colloidal silica ; there are 
indications from the microscopic sections that this separation did occur round the 
plants in the matrix in the first instance. On this view the silicification, or at 
least its early stages, would have been progressive as the bed was accumulating, 
instead of the thick peat-bed being first formed and then at a later date silicified 
as a whole. Such an explanation would account for the loose packing and perfect 
preservation of the plants in the basal beds where, on the analogy of recent peats, 
the greatest alteration and decay would have been anticipated. 
The peculiar changes which are involved in the development of hemispherical 
projections in Rhynia Gwynne-Vaughani and the necrosis areas, wound-reactions 
and unequal enlargement of cells in the stems of this species and of R. major (see 
Part IV, pp. 835-6), must be recalled here. These various developments suggested 
a reaction to some prolonged external stimulus. The possibility that this was 
connected with the volcanic conditions that accompanied the supply of the siliceous 
water appears to be a justifiable speculation to raise. The intumescences and other 
reactions that are known to follow on the stimulus of irritating vapours in the 
case of existing plants (Part IV, p. 834) should be borne in mind in considering this. 
The characters of the deposit as a whole appear to indicate a steady growth 
and accumulation on a subsiding surface. This might have been a swamp beside 
* Of. Schimper, Plant Geography , p. 386, and literature there cited. 
TRANS. ROY. SOC. EDIN., VOL. LII, PART IV (NO. 33). 
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