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, however, 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. CHAPTER VIII PAST HISTORIES OF PLANT FAMILIES I. Flowering Plants, Angiosperms In comparison with the other groups of plants the flowering families are of recent origin, yet in the sense in which the word is usually used they are ancient indeed, and the earliest records of them must date at least to periods hundreds of thousands of years ago. Through all the Tertiary period (see p. 34) there were numerous flowering plants, and there is evidence that many families of both Monocotyledons and Dicotyledons existed in the Upper Cretaceous times. Further back than this we have little reliable testimony, for the few specimens of so-called flowering plants from the Lower Mesozoic are for the most part of a doubtful nature. The flowering plants seem to stand much isolated from the rest of the plant world; there is no _direct_ evidence of connection between their oldest representatives and any of the more primitive families. So far as our actual knowledge goes, they might have sprung into being at the middle of the Mesozoic period quite independently of the other plants then living; though there are not wanting elaborate and almost convincing theories of their connection with more than one group of their predecessors (see p. 108).