118 G. R. Wieland — Cycadeoidean Flower-hud Struct 



ure. 



phologist bent on making the fullest use of the paleontologic 

 record can afford to ignore. The} r do suggest a means of sim- 

 plifying our conception of the evolution of the early land 

 forests ; and they do go far to show how a great variety of 

 floral structures could have been rapidly developed in late 

 Silurian and earlier Devonian land plants, a variety permitting 

 development of later types of plants by a frontal or abreast 

 course of development along more or less parallel lines. And 

 certainly such a conception of the main course of plant evolu- 

 tion is more consonant with general facts of distribution and 

 theories of climatic change than is any view of descent by a 

 series of dichotomies. 



There is, in short, a broad suggestion of parallelism and 

 periodicity in plant evolution. It has hitherto been difficult to 

 reconcile the meager record of Silurian plants with the sudden 

 appearance of varied Devonian types well advanced and so 

 persistently lacking in forerunners. But this paucity of record 

 is more easily accounted for if the direct evolution of new 

 structures be held to have reached a climax in the first rapid 

 covering of the land by homosporous types. From the leafy 

 crowns of these earliest of all forest forms must in time have 

 been derived the early heterosporous types, and from these in 

 turn by continued sterilization emplacements, fusions, reduc- 

 tion and branching, a profusion of early seeds and thea flowers. 

 Such would be, hypothetically speaking, the line of least resist- 

 ance in early plant evolution. We know that evolution of the 

 early land plants may well have been rapid just as many phyla 

 of invertebrates develop diversity of form early in their history, 

 and just as in recent geologic time where islands have been 

 freshly populated plant forms elsewhere herbaceous quickly 

 develop variety of species and tree-like stature. 



That ancient seeds then have such a leafy structure appears 

 to find the simplest explanation in the fact that a seed is as 

 much a branch or derivation of a crown of fertile fronds as 

 flowers have been conceived to be. And the striking character 

 of the staminate disk of Cycadeoidea raises the question 

 whether in the sporophyll fusions and reductions resulting in 

 seed coats, whorls of sterilized microsporophylls, or of bracts, 

 may not in some forms be merged as seed coat components, it 

 being clearer now than ever that no one course of evolution 

 can account for all the varied types of seed integuments. In 

 some such manner the double nature of the integument in 

 those forms where there is a series of inner testal bundles 

 separated from an outer by the middle stone might be 

 accounted for. 



Finally, it is clear that if seed and flower have this dual 

 homology and were thus successively derived from primitive 



