A photograph of an actual section of such a cone, cut slightly obliquely through the length of the axis, is seen in [fig. 110], where the upper groups of sporangia are cut tangentially, and show their grouping round the sporophyll to which they are attached.
A few single tetrads of spores are enlarged in [fig. 111], where it will be seen that the large spores are of a similar size, but that the small ones of the tetrads are very irregular. They are aborting members of the tetrad, and appear to have been used as food by the other spores. In each sporangium large numbers of these tetrads develop and all the ripe spores seem to have been of one size.
In a species of Calamites (C. casheana), otherwise very similar to the common one we have been considering, there is a distinct difference in the sizes of the spores from different sporangia. The small ones, however, were only about one-third of the diameter of the large ones, so that the difference was very much less marked than it was between the small and large spores of the Lycopods.
Among the palæozoic members of the group are other genera closely allied to, but differing from Calamites in some particulars. One of these is Archæocalamites, which has a cone almost identical with that of the living Equisetums, as it has no sterile bracts mingled with the umbrella-like sporophylls. Other genera are more complex than those described for Calamites, and even in the simple coned Archæocalamites itself the leaves are finely branched and divided instead of being simple scales.
But no genus is so completely known as is Calamites, which will itself suffice as an illustration of the palæozoic Equisetaceæ. Though the genus, as was pointed out above, shows several important characters differing from those of Equisetum, and parallel to some extent to those of the palæozoic Lycopods, yet these features are more of a physiological nature than a systematic one, and they throw no light on the origin of the family or on its connection with the other Pteridophytes. It is in the extinct family dealt with in the next chapter that we find what some consider as a clue to the solution of these problems.
Fig. 111.—Tetrads of Spores of Calamites
S, Normal-sized spores; a, b, &c., aborting spores.
CHAPTER XVI
PAST HISTORIES OF PLANT FAMILIES
IX. Sphenophyllales
The group to which Sphenophyllum belongs is of considerable interest and importance, and is, further, one of those extinct families whose very existence would never have been suspected had it not been discovered by fossil botanists. Not only is the family as a whole extinct, it also shows features in its anatomy which are not to be paralleled among living stems. Sphenophyllum became extinct in the Palæozoic period, but its interest is very real and living to-day, and in the peculiar features of its structure we see the first clue that suggests a common ancestor for the still living groups of Lycopods and Equisetaceæ, which now stand so isolated and far apart.