78 ANCIENT PLANTS 



foliage leaves. They were in this on a level with the 

 simple sporangia of ferns which appear on the backs of 

 the fronds, a fact which is of great significance both for 

 our views on the evolution of seeds as such, and for the 

 bearing it has on the relationships of the various groups 

 of allied plants. This will be referred to subsequently 

 (Chapter XI), and is mentioned now only as an example 

 of the difference between some of the characters of early 

 fossils and those of the present day. 



It is true that botanists have long recognized the 

 organ which bears seeds as a modified leaf. The carpels 

 of all the higher plants are looked on as homologous with 

 leaves, although they do not appear to be like them 

 externally. Sometimes among living plants curious 

 diseases cause the carpels to become foliar, and when 

 this happens the diseased carpel reverts more or less to 

 the supposed ancestral leaf-like condition. It is only 

 among the ancient (but recently discovered) fossils, how- 

 ever, that seeds are known to be borne normally on 

 foliage leaves. 



From Mesozoic plants we shall learn new conceptions 

 about flowers and reproductive inflorescences in general, 

 but these must be deferred to the consideration of the 

 family as a whole (Chapter XIII). 



Enough has been illustrated to show that though 

 the individual cells, the bricks, so to speak, of plant 

 construction, were so similar in the past and present, 

 yet the organs built up by them have been continually 

 varying, as a child builds increasingly ambitious palaces 

 with the same set of bricks. 



