1902] 



EVOLUTION OF VASCULAR TISSUE OF PLANTS 



221 



as has been practically proved by the occurrence of obvious ves- 

 tiges of the concentric type of structure, such as prevails among 

 the MeduUoseae, both in the vegetative stems and in the pedun- 

 cles of all the pluricylindric forms of modern cycads. 



In the Coniferae, from the yew tribe upward to the pines, as 

 also in the Gnetaceae, all trace of 

 the primitive mesarch type of bundle 

 has disappeared from the stem, and 

 we find the endarch type, with pro- 

 toxylem impinging directly on the 

 pith, universally prevailing. That 

 ancient Devonian and Carboniferous 

 plant, Cordaites, and our modern 

 maidenhair tree, Giiikgo biloba, 

 assumed also, in accordance probably 

 with the exigencies of their arboreal habit, the endarch type of 



ph 



Fig. 7. — Concentric bundle of 



leaf-stalks. 



bundles in the stem. But Pitys aniiqria, P. primaeva, Calamopitys 

 fascicnlaris, C. beiiiertiana, and Dadoxylon Spe?iceri, forms perhaps 

 allied to Cordaites, still retain the older mesarch type.' 



As regards the foliar organs of all of these plants we have 

 been considering, it is found that the primitive character of the 

 vascular structure is in them much more persistent than it is in the 

 stem. The concentric fern-type of bundle is still present in the 

 leaf-stalks of Lyginodendron (Rachiopteris) and Heterangium, 

 that of the former plant consisting of a solid stele with three or 

 more protoxylem groups at its periphery, the whole being sur- 



{A 



In Medullosa, Cordaites, and mod- 



ern cycads the bundles of the leaves are collateral and mesarch 

 in structure ; as are also those of the foliage leaves of conifers, 

 although here the centripetal primary xylem is either greatly 

 reduced or modified to form a quasi-new structure, the transfu- 



sion tissue. 



In Ginkgo this reduction also obtains in the foliage 



leaf. But in the most primitive vegetative foliar organs of the 

 plant, the cotyledons, both in Ginkgo and Cephalotaxus (the 



7 Scott, D. H., On the primary structure of certain palaeozoic stems with the 

 Dadoxylon type of wood. Trans. Roy. Soc Edinburgh 40: [pt. II]. 1902. 



