OH. XXXVj PALAEOZOIC SEEDS 301 



directing attention to data bearing on evolutionary problems. 

 The chief interest of Palaeozoic seeds to the botanist lies in the 

 facts they contribute towards the elucidation of questions con- 

 nected with the promotion of the megasporangium and megaspore 

 of the Pteridophyta to the higher stage represented by the integu- 

 mented megasporangium (nucellus) and single megaspore that in 

 the main fulfil the definition of a seed^. 'With the evolution of 

 the seedj' as Oliver says, 'the plant rose at a bound to a higher 

 plane, and this structure in its perfected form has become the 

 very centre of the plant's existence^.' We can as yet form a very 

 partial conception of the successive stages in the adoption of the 

 seed-habit, but since 1855, the year in which Hooker and Binney* 

 pubUshed their paper on the structure of Trigonocarfus, ample 

 proof has been furnished of the importance of Palaeozoic seeds 

 from the standpoint of affinity between recent Gymnosperms 

 and extinct seed-bearing plants, and of the modus operandi of 

 evolutionary tendencies. A cursory examination of Palaeozoic^ 

 seeds suffices to demonstrate their resemblance to those of recent 

 Cycads and the seed of Ginkgo biloba ; but while it is clearly with 

 these Gymnosperms that the majority of the seeds described in 

 the following pages are most closely allied, the extinct types 

 possess many distinguishing features that throw light on some 

 at least of the factors concerned in the production of the modern 

 type. In many of the Palaeozoic seeds the nucellus stands free 

 within the integument, to which it is attached only in the chalazal 

 region, in contrast to the lateral union between integument and 

 nucellus in the ovules of recent Cycads. It has been suggested 

 by Oliver* that the seed of the Conifer Torreya affords a clue to 

 the interpretation of this difference and that the lower part of 

 the seed in Cycads and Torreya represents a later intercalation in 

 the basal portion of the ovule, the ancient type having a free 

 nucellus in contrast to the nucellus of modern seeds which is free 

 only at the apex. It has been objected^ that there are no adequate 

 grounds for assuming the addition of an intercalated zone or of 

 the elongation of the ovule that this impUes, the more probable 



1 See Vol. n. p. .56. ^ OHver (06) p. 726. 



» Hooker and Binney (55). * Oliver (03). 



6 WoradeU (05) p. 58; Salisbury (14) p. 67. 



