ORGANIZATION OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 911 
sions, the diameter of the smallest sister-cell being often scarcely a quarter that of 
the largest. Sometimes three out of the four remain quite small, and occasionally an 
entire tetrad seeins to have been abortive (see the figures cited ; in fig. 34, B and C, 
one spore in each tetrad is of relatively minute size). 
Such very minute spores are also frequently found, among the normal ones, in 
sporangia in which the spores are already isolated from one another. If these latter 
eases stood alone, we might have doubted whether the small spores were not foreign 
bodies ; their presence, however, as members of the tetrads, within the mother-cell- 
wall, proves conclusively that they are really sister-cells which have remained behind 
the others in their development. The phenomenon is extremely frequent, as is 
sufficiently indicated by the figures. 
We do not think there is any reason to doubt that four spores were regularly 
formed within each mother-cell, as is constantly the case in all recent vascular 
Cryptogams. It seems, however, to have been the exception for all four to be 
equally developed. , 
This frequent abortion of many of the spores in a sporangium, so frequent that we 
cannot but regard it as a normal process, seems to us to be a fact of extreme interest. 
We cannot doubt that Calamostachys Binneyana was a homosporous form. The 
large number of specimens available for investigation, many of them including all 
parts of the strobilus, establishes the strongest presumption that macrospores must 
have been found, if they existed. 
We know, however, that the closely similar species, C. Casheana, was hetero- 
sporous, its microspores not being very different in size from the normal spores of 
C. Binneyana. When we come to describe the heterosporous species, we shall find 
strong reason to believe that in its macrosporangia a constant abortion of some of the 
spores went on. 
We would suggest the hypothesis that the abortion of certain of the spores, and 
the consequent increased nutrition of their surviving fellows, may have been the 
physiological condition which ultimately rendered possible the development of 
specialized macrospores. 
We know that among existing heterosporous vascular Cryptogams the abortion, 
either of the majority of the mother-cells (Ligulatz), or, in addition, that of the 
sister-cells of the macrospore (Rhizocarpez), is a constant accompaniment of the 
favoured development of the surviving spore or spores. 
We know also that in Equisetum, which is homosporous, but in which the prothalli 
are regularly dicecious, it depends upon the nutrition whether a spore develops into 
a male or female prothallus. The better fed prothalli become female, the worse fed, 
male.* 
In Calamostachys we have the intermediate conditions. In C. Binneyana we find 
the beginning of spore-abortion, involving improved nutrition of the surviving spores. 
* Bucarigy, “ Entwickelung des Prothalliums von Equisetwm,” ‘ Bibliotheca Botanica,’ 1887, 
MDCCCXCIV.—B, 6A 
