•-■ ^x ^ 



THE EVOLUTION OF THE VASCULAR TISSUE 



OF PLANTS. 



W. C. WORSDELL. 



(with seven figures) 



It is a sign of the times and of the natural progress which 

 botanical science is making that the evolution of the more 

 deeply seated part of the organization of plants is beginning 

 to receive attention. The most vital and important part of the 

 anatomical structure is doubtless that of the vascular system, 



and it is the development of this 



1/ upward through some of the most 



important groups of plants of which 



I propose to take a general survey. 



We appear to be justified in 



#1"""^ looking for the primary origin of 



the complex vascular structure of 

 the stems of cormophytes in the 

 single solid stele, the protostele, con- 



Fig. I.— Theprotostele;x, xylem; sisting of a central mass of wood, 



/jr, protoxylem ; /y^, phloem. , j j u ^ ^^r»^ nf 



*^ or xylem, surrounded by a zone or 



bast, or phloem (yf^ /), this in its turn having been derived from 

 the primitive conductive tissue of the sporophytic stem of some 

 bryophytic ancestor. If we take the great group of the ferns 

 this structure always appears as the first stage in the individual 

 development of each type, as has been shown by the researches 

 of Leclerc duSablon* and others. This is also the type found in 

 the mature vascular structure of several of the most primitive 

 ferns, such as the Hymenophyllaceae, Lygodium (Schizaeaceae), 

 and Gleichenia, among modern forms, and Botryopteris among 

 fossils. 



*Sablon, Leclerc du, Recherches sur la Tige des Foug^res, Ann. Sci. Nat. 

 VII. II : — , 1890. 



Bot. VII. 11: 



216 [SEPTEMBER 



4 



/nW'ff 



k 



t 



