The strobilus axis agrees in structure with that of _C. Binneyana_, but in _C. Casheana_ a band of secondary xylem forms the peripheral portion of the triangular stele. Were any further proof needed of the now well-established fact that secondary growth in thickness is by no means unknown as an attribute of Vascular Cryptogams, the co-existence in the same cone of a cambium layer producing secondary wood and bark, and cryptogamic macrospores and microspores, affords conclusive evidence[712]. The dogma accepted by many writers for a considerable number of years that the power of secondary thickening is evidence against a cryptogamic affinity, has been responsible for no little confusion in palaeobotanical nomenclature. On the axis of _Calamostachys Casheana_ there are borne alternate whorls of fertile and sterile appendages similar to those in the homosporous _C. Binneyana_, but they are inclined more obliquely to the axis of the cone. Macrospores and microspores have been found in sporangia borne on the same sporangiophore. [Illustration: FIG. 96. _Calamostachys Casheana_ Will. A sporangium with macrospores and abortive spores. × 65. (After Williamson and Scott.)] The spore-tetrads in the macrosporangia occasionally include aborted sister-cells like those noticed in _C. Binneyana_; this phenomenon is well illustrated by the unequally nourished spores in the sporangium of fig. 96, but no such starved spores have been found in the microsporangia. In this cone, then, heterospory has become firmly established, but the occurrence of undersized spores in a macrospore-tetrad leads us back to the probable lines of development of heterospory, which are seen in _C. Binneyana_ at their starting-point. In the two species of strobili which have been described, _Calamostachys Binneyana_ and _C. Casheana_, the sporangiophores or sporophylls are given off at right angles to the axis, and midway between the sterile whorls. These are two of the most important distinguishing features of the Calamitean cones included under the generic term _Calamostachys_. In another form of cone, which also belongs to Calamitean stems, the sporangiophores arise in the axil of the sterile leaves, and are inclined obliquely to the axis of the cone. To this type the generic name _Palaeostachya_ has been applied by the late Prof. Weiss[713] of Berlin. The portion of a cone shown in fig. 97 shows the arrangement of the sterile and fertile appendages characteristic of _Palaeostachya_. [Illustration: FIG. 97. _Palaeostachya pedunculata_ Will. Part of a cone, × 3. (After Weiss.)] It is practically impossible to distinguish between cones of the _Calamostachys_ and _Palaeostachya_ type in the case of imperfectly preserved impressions; indeed we cannot assume that all long and narrow cones with spirally disposed verticillate bracts are Calamitean. We must have the additional evidence of internal structure or of the direct association of the cones with Calamitean foliage.