On the spores of some Carboniferous plants; their development
Authors: Moore Leslie Rowsell 
Published: October 2009

Summary
During the investigation of microspore yields from coal-seam analyses, some difficulty was found in assessing the intrinsic value of the variations shown by spores about those arbitrary "spore types" described and figured by Raistrick and Simpson (1933), and Raistrick (1934, 1937). Raistrick himself stated quite clearly that his classification of the microspores should be regarded as entirely arbitrary; that the groups of spores were chosen generally for purposes of identification, and that no affinity was implied by the classification adopted. It therefore appeared advisable to obtain further information as to the type of spores yielded by the known species of fossil plants, and furthermore to consider how far the relatively mature spores from the same fossil plant varied amongst themselves, both in size and ornamentation. Accordingly a method was devised which enabled large numbers of spores to be obtained from the fructifications of fossil plants of Carboniferous age.

Wherever possible fructifications were studied which were attached to a plant, thus enabling the spores obtained to be referred to their parent plant. Certain other fructifications were merely associated with identifiable plant material, and therefore no direct reference to a parent plant could be inferred. Nevertheless, the spore yield from such a fructification was important in so far as it represented an assemblage of spores from one plant. Some 30 fructifications were studied, and these included representatives of the Pteridospermeae and Filicales, Lycopodiales, Equisetales, and Sphenophyllales. At the same time, certain organs which superficially resembled the seeds
Carpolithus
and
Cordaicarpus
also yielded abundant spores.

In many cases during this investigation the spores from one plant have been observed to show considerable diversity in size, form, and ornamentation, and it is believed that this diversity should be ascribed principally to the differing stages of maturity exhibited by these spores.

Accordingly the spore development for a plant has been studied, and comparisons of the developments for a number of plants have indicated that certain supposed "spore types" are merely developmental stages of a single species. The following examples, which represent a selection from others recognized during the investigation, serve to show the nature of this transition from one supposed "spore type" to another during the development of a spore. Thus immature spores of a "spore type" D exhibit intermediate stages during development similar to "spore types" A
1
and A
3
, and finally, when relatively mature, show the characteristics of "spore type" A
5
. Other immature spores similar to "spore type" D develop into a relatively mature spore of the "spore type" C. From immature spores of the "spore type" B development may proceed through the intermediate stages of "spore types" D to a mature spore of "spore type" C; alternatively from "type" B there may be derived through intermediate stages spores of the "types" E
2
and E
3
.

It follows that the existing and arbiwary classification of spores into "spore types" is fundamentally unsound, and it is necessary to consider whether the use of some of these "spore types" in coal-seam analyses may lead to erroneous conclusions regarding the correlation of coal seams. The range of variation, both in size and ornamentation, for relatively mature spores was found to be considerable, and in certain cases some assessment of this range has been made.
The work has shown that the development of a mature spore follows certain well-defined lines, and that it is possible to indicate from this development the affinities of many spores isolated by coal-seam analyses. By this means it may be possible ultimately to institute a true classification of spores, founded upon a sound biological basis of affinity between the spores themselves.