xxxv] 



PALAEOZOIC SEEDS 



303 



pc. 



nature of the vascular supply in this generalised type and in indi- 

 vidual genera should be compared with that in the seeds of recent 

 Cycads 1 described in Chapter xxvin. 



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 matura- 

 tion of the microspores. A com- 

 parison of the genera Physostoma, 

 Lagenostoma, Conostoma, Trigo- 

 nocarpus. Stephanospermum and 

 others reveals the occurrence of 

 very different though not unrelated 

 structural features especially in the 

 apical region of the seed, 

 seeds, including Physostoma pro- 

 bably 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. 



Among the numerous types of Palaeozoic seeds are several 

 which invite comparison with the fruits or carpels, apart from 

 the seeds, of Angiosperms. Impressions of Samaropsis seeds 

 (figs. 502, B K; 503; 504) bear a close resemblance to the 

 laterally expanded fruits of the common Crucifer Thlaspi arvense ; 

 the ribbed testa of Hexagonocarpus (fig. 506, H) and other genera 



1 See page 25. 



mi 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.) 



