the main strand gives off a pair of bundles in the sarcotesta in the principal plane, as in _Cardiocarpus_[799]; from the tracheal mass in the chalazal region numerous bundles pass up the nucellus as far as the floor of the pollen-chamber. The nature of the vascular supply in this generalised type and in individual genera should be compared with that in the seeds of recent Cycads[800] described in Chapter +xxviii.+ [Illustration: Fig. 492. ‘A conjectural synthetic type of seed embodying the characters of such a seed as _Stephanospermum_ with those of a _Cardiocarpus’_; _n_, nucellus; _pc_, pollen-chamber. (After Oliver.)] Recent research has revealed the not unexpected fact that in such Upper Carboniferous petrified seeds as have been investigated—a small proportion of the large number produced in the Palaeozoic forests—there was a remarkable range in the mechanism connected with pollination and the maturation of the microspores. A comparison of the genera _Physostoma_, _Lagenostoma_, _Conostoma_, _Trigonocarpus_, _Stephanospermum_ and others reveals the occurrence of very different though not unrelated structural features especially in the apical region of the seed. These seeds, including _Physostoma_, probably the most archaic type, represent a stage in evolution already far removed from the starting-point: the diversity of plan recalls the variety in the form of the chloroplasts in the Green Algae, and in both cases we are in touch with an experimental phase representing a tentative advance towards greater efficiency. In its differentiation into an outer fleshy region, the sarcotesta, a stony layer, the sclerotesta, and in many cases an inner flesh, the Palaeozoic seeds resemble recent Cycads: in both extinct and modern seeds the balance of evidence would seem to be in favour of attributing a single rather than a double origin to the integument.