302 GYMNOSPEKMOTJS SEEDS [CH. 



view being that the lateral union of nucellus and integument 

 represents congenital fusion in the ancestral type, a fusion com- 

 parable with that of the coherent petals of a gamopetalous corolla. 

 In the presence of a pollen-chamber most of the Palaeozoic seeds 

 agree with those of recent Cycads, but in the extinct forms it is 

 usually a more highly developed structure. The name pollen- 

 chamber was given by Brongniart^ to the pollen-containing 

 cavity in the free region of the nucellus in the petrified seeds from 

 St ]5tienne in ignorance of the use of the same term by Grifiith^ 

 in a posthumous work pubhshed in 1852 for the nuceUar chamber 

 in Cycas^. The genus Stephanospermum (fig. 494, A) illustrates 

 the means by which the pollen-chamber was liberally supplied 

 with water and thus adapted to the requirements of fertilisation 

 by motile gametes. The pollen-chamber and its vascular supply 

 paved the way for siphonogamy, that is the development of a 

 pollen-tube for the more direct transmission of the male sperms. 

 J The highly developed mantle of tracheal tissue at the periphery 

 of the nucellus in Stephanospermum, represented on a reduced 

 scale by the separate vascular strands of other seeds, may be 

 compared with the tracheal investment to the nucellus in the 

 recent Dicotyledonous genus Cassytha*. The presence of a 

 nucellar vascular system in several Palaeozoic seeds is a feature 

 in which they differ from those of recent Cycads with the exception 

 of Bowenia. The retreat of the vascular supply from the imme- 

 diate neighbourhood of the pollen-chamber in recent Cycads may, 

 as Ohver points out, be correlated with the evolution of the pollen- 

 tube — the substitution of siphonogamy for zoidiogamy. The 

 diagram reproduced in fig. 492 represents a synthetic type based 

 on such seeds as Stephanospermum and Cardiocarpus which 

 illustrate an arrangement of conducting tissue frequently found 

 in Palaeozoic seeds : the main strand gives off a pair of bundles 

 in the sarcotesta in the principal plane, as in Cardiocarpus^ ; from 

 the tracheal mass in the chalazal region numerous bundles pass 

 up the nucellus as far as the floor of the pollen-chamber. The 



' Brongniart (81) p. 31. 



^ For an account of the work of this author see Lang in Oliver (13) p. 178 

 3 See Scott (09) B. p. 543. 4 Mirande (05). 



* For definition of Cardiocarpus, see page 338. 



