May 22, 1896.] 



SCIENGE. 



759 



ploxylon, especially by tlie marked diminu- 

 tion in the diameter of the primary xylem, 

 exhibit the secondary growth in roots, 

 stems and leaf bases, but only as an elemen- 

 tary stage in the leaf itself. 



Representing the present Brongniartian 

 School, M. Eenault cites the somewhat 

 anomalous Poroxrjlon group (' although be- 

 longing rather to another series leading to 

 the Conifera') as examples showing the 

 double growth in roots, stems and leaves, 

 predicting that their still unknown fruits 

 will probably be found to be small seeds con- 

 structed on the plan already observed in 

 the contemporaneous Gymnosperms. If so, 

 the Poroxylon will be especially exemplary 

 in combining the characters of Phanerogams 

 and Cryptogams. The ' libero-ligneous ' 

 bundle of the leaf has the double structure 

 in Colpoxylon, while the structure is simple 

 in Medullosa, a genus allied to the Cycads, 

 though both have lost all traces of their 

 centripetal wood, except some vascular 

 bundles scattered through the pith, the 

 woody element of the stem being composed 

 of tracheids punctate in many rows and 

 medullary rays organized like those of the 

 Cycads. He concludes that the Phanero- 

 gamic characters became gradually associ- 

 ated with the Cryptogamic, increasing little 

 by little to preponderancy and finally ex- 

 terminating the latter ; that these changes 

 are successively accomplished in the prin- 

 cipal organs of the plant and in a definite 

 order, the fruits being last to change. In 

 effect, M. Eenault suggests that the differ- 

 ence between the Paleozoic Lycopod group 

 and the living Cycad is hardly more than 

 that between the living Cycad and the 

 typical Phanerogam. David White. 



THE EMBANKMENTS OF THE BIVEB PO. 



Theee is probably no part of the world 

 in which the action of rivers in carrying 

 and depositing sediment can be better seen 

 and more readily studied than in the plains 



of Lombardy and along the adjacent shores 

 of the Adriatic, and no district has con- 

 tributed more to our knowledge of the im- 

 portant subject of river action and delta 

 building than has this portion of Northern 

 Italy. 



In this well settled country the very 

 rapid advance of the land upon the sea 

 everywhere has been especially remarked 

 and could not escape the attention of the 

 most unobservant, since, as is well known, 

 the very town of Adria, which gives its 

 name to the Adriatic Sea and which was a 

 sea port in the time of Augustus, now lies 

 14 miles inland. 



One statement concerning the chief of 

 these Lombard Elvers, the Po, taken fi'om 

 chapter eighteen of Lyell's Principles of 

 Geology, has been copied and recopied in 

 one generation of text-books after another, 

 a statement so remarkable that wherever 

 met with it always arrests one's attention. 

 It is that in which, after speaking of the 

 action of the dykes, between which these 

 Lombard rivers are confined in causing 

 a portion of the sediment, which would 

 otherwise be spread over the plains by the 

 annual inundations, to settle in the bottom 

 of the river channel, with the consequent 

 necessity of from time to time increasing 

 the height of the dykes, he says, " Hence it 

 happens that these streams now traverse 

 the plains on the top of high mounds, like 

 the water of aqueducts, and at Ferrara the 

 surface of the Po has become more elevated 

 than the roofs of the houses." 



On reading this passage one cannot but 

 tremble for the fate of the city should the 

 river break through its dykes, as it has al- 

 ready done on several occasions, and, being 

 precipitated into the city, tear its way 

 headlong to the sea. 



A visit to Ferrara toward the end of May 

 last served, however, to show that this 

 danger is less imminent than might be sup- 

 posed from Lyell's description. 



