136 SCIENCE PROGRESS. 



ing because it shows that when it was written Van Tieghem 

 had no idea of a central cylinder in the stem. 



Two years later, however, in 1872, in describing (2) 

 the transition from root to shoot in Tagetes patiila, he 

 writes how the " membrane protectrice " (endodermis) is 

 continued up into the stem, retaining its characteristic 

 thickenings, and immediately internal to it the "membrane 

 rhizogene" of the root (later named the pericycle) is also 

 found in the stem still giving rise to rows of lateral roots, 

 one row arising from each interval between two bundles. 

 Opposite the bundles, however, he holds that the "membrane 

 rhizogene" is interrupted, since here the endodermis abuts 

 directly on the group of fibres capping the bundle, fibres 

 which in accordance with the current opinion he considered 

 to belong to the phloem. Here then we have the first 

 clear description of the continuation of the central cylinder 

 of the root into the stem, and the idea of this continuation 

 is the fundamental idea of the stelar theory. It is most 

 clearly expressed in a note on p. 112, " Ainsi, et j'insiste 

 sur ce point, la tige est, comme la racine, et dans toute son 

 etendue, composee d'un cylindre central et d'un parenchyme 

 cortical limite en dehors par un epiderme, en dedans par 

 une membrane protectrice ou endoderme ". 



The generality of this condition is further insisted upon : 

 " Le caractere sur lequel je viens d'appeler l'attention se 

 retrouve dans la tige de la grande majorite des plantes 

 vasculaires, mais il souffre pourtant quelques exceptions. 

 M. Caspary a montre, en effet, que dans quelques plantes 

 (Minyanthes trifoliata, Adoxa moschatellina, Bi-asema 

 peltatd) chaque faisceau constitutif de la tige est indivi- 

 duellement entoure par une membrane protectrice a cellules 

 plissees (' Bemerkungen liber die Schutzscheide,' in Pring- 

 scheims Jahrbucher, 1865-66, iv., p. 10 1). J'ai retrouve le 

 meme fait sur quelques autres plantes, notamment sur 

 r Hydrocleis Humboldtii. Dans ce cas, il n'y a pas non 

 plus de membrane rhizogene dans les entrenceuds de la 

 tige, et il n'existe aucune solution de continuity aucune 

 distinction reelle entre le parenchyme cortical et la moelle ' 

 (p. 113). This paragraph shows clearly that thus early 



