THE STELAR THEORY; A HISTORY AND A 



CRITICISM. 



PART II. 

 THE METAMORPHOSES OF THE STELE. 



IT is clear that the theory of polystely forms an 

 integral part of the general stelar doctrine, and we 

 can hardly refuse to accept its main idea. But though each 

 stele in the polystelic stems of, for instance, Aiiricula Ursi 

 and many Polypodiacea^ is clearly the equivalent of the 

 whole cylinder in the hypocotyl of the same plants, cases 

 exist in which we seem forced to consider as steles, 

 vascular strands which have none of the characters of the 

 cylinder left about them. 



Deriving our idea of the typical stele from the mono- 

 stelic organ, we come to consider it as essentially cylindrical 

 and radially symmetrical. It is true that diarch roots are 

 bilateral in structure, and the primary root and hypocotyl of 

 very many ferns being diarch the steles of a great number 

 of their stems are likewise diarch and hence bilateral. And 

 this bilaterality often extends to the shape of the stele which 

 becomes oval or band-shaped instead of circular in transverse 

 section, the two protoxylems being situated at the extremi- 

 ties of the figure. Another step is for the stele to become 

 more or less semilunar in transverse section, so that it is no 

 longer symmetrical about the plane passing through the 

 protoxylems, but only about the bisecting plane perpendi- 

 cular to this. And further the protoxylems may lose their 

 symmetrical arrangement, or one only may be present, and 

 this may be excentrically placed (Angiopteris). We clearly 

 could not tell that such strands were steles if we had no 

 knowledge of their connexions and disposition. At least 

 as far as tissue arrangement goes they may often be said to 

 have lost those characters which entitle them to the name. 

 A similar difficulty meets us in the case of the vascular 

 strands in many fern leaves. Undoubted steles found in 



