XXXV] PRESERVATION OF SEEDS 305 



living plants; but even at the risk of allowing speculation too 

 free a hand the attempt is worth making, and it may be urged 

 that, as in phylogenetic enquiries so in other branches of botany, 

 facts obtained from plants of other ages may serve to supply 

 deficiencies in knowledge based only on existing forms. One of 

 the difficulties inseparable from the study of fossil plants, namely 

 the identification of impressions and casts with specimens ex- 

 hibiting anatomical characters, is particularly well illustrated 

 by seeds. The description of a genus based on mere external 

 form may sometimes be extended without great risk of error to 

 include species founded on anatomical characters, but on the 

 other hand, there are many instances in which — despite a general 

 resemblance in form and size between petrifactions and impres- 

 sions — lack of evidence of generic identity requires the employment 

 of distinctive names. The determination of impressions is, as 

 Lesquereux recognised, 'subject to a great deal of uncertainty,' 

 and many of the genera founded on external features are purely 

 artificial and include species that have no essential features in 

 common. Moreover in the case of petrified specimens the apparent 

 absence of an external fleshy layer is often due to destruction 

 before preservation : as Solms-Laubach^ points out, it is obviously 

 impossible to be certain as to the number of integumental layers 

 in seeds that are not well preserved in all their parts. Goeppert 

 founded a new genus, Acanthocarpus, on a Permian seed described 

 as A. xanthioides^, because of the occurrence of spinous processes 

 attached to an obcordate kernel : these apparent spines are in 

 all probabihty the remains of a very imperfectly preserved sarco- 

 testa. The preservation of the central portion of a seed, that 

 is the seed-cavity with the enclosing shell, conveniently called 

 the nucule, has often led to an unnecessary multiphcation of 

 generic terms. Other examples of confusion resulting from 

 different states of preservation are quoted in the accounts of 

 some of the selected types. 



Wilhamson in 1877 pointed out that we learn from the large 

 number of different kinds of Palaeozoic seeds that 'there were 

 in the Carboniferous forests many gymnospermous stems clothed 



1 Soln^s-Laubaoh (91) A. p. 118. 



2 Goeppert (65) p. 177, PI. xxvi. figs. 27, 28. 



S. in 2a 



