2 20 HEREDITY AND EVOLUTION IN PLANTS 



The gap between the stamen of Cycadeoidea and the 

 type characteristic of modern Angiosperms is partially 

 bridged by the genus Williamsonia (which has simple vs. 

 pinnately compound stamens), and by another genus, 

 Wielandiella, both older genera than Cycadeoidea (Fig. 

 103). From this it has been inferred that the Hemicycad- 

 ales are a lateral branch, further removed than their 

 ancestors from the direct evolutionary stock of the 

 Angiosperms. 



146. Origin of Dicotyledony. — Two problems of major 

 importance are involved in the question of the evolution 

 of Angiosperms, namely, the origin of dicotyledony and 

 the origin of monocotyledony. Are dicotyledons more 

 ancient than monocotyledons, or vice versa? Again, in 

 the evolution of seed-bearing plants was the condition of 

 polycotyledony antecedent to that of dicotyledony, or the 

 reverse? This would be a comparatively easy question 

 to answer if we had an unbroken series of fossil remains 

 of the primitive and intermediate spermatophytes; but 

 unfortunately such evidence has not yet been discovered. 

 We know nothing of the embryos of the geological ances- 

 tors of modern conifers. The Mesozoic gymnosperms 

 {Cycadeoidea and other related genera) are known to have 

 had dicotyledonous embryos, but these forms do not stand 

 in the ancestral hne of the (polycotyledonous) conifers 

 of to-day. To answer our question, therefore, we must, 

 for the present, depend largely on the study of living forms. 

 The evidence has seemed conflicting, and for nearly three- 

 quarters of a century opinion has varied. Adanson and 

 Jussieu, in the early nineteenth century, contended that 

 polycotyledony was derived from dicotyledony by a spUt- 

 ting of the primordia of two original cotyledons; Sachs 



