THE STELA R THEORY. 225 



mands for the supply of vascular tissue to leaves, the plant 

 either responded by increasing the bulk of the stele and 

 multiplying the number of its vascular strands, or by sub- 

 stituting a number of simple steles for the original single 

 one. This last occurrence happened once at least in the 

 Pteridophyta (probably more often), and more than once 

 among the Phanerogams. 



The primordial stele is represented at the present day 

 by the single sharply defined stele of the embryo, which is 

 maintained in the root and hypocotyl, and which passes over 

 in the stem to one of the modern types of structure, 

 necessary to the various demands of the leafy shoot. The 

 arrangements at the apex of the latter are naturally adapted 

 to form the particular type of structure in question, and 

 can in no case be considered as representing an ancestral 

 form. 



BIBLIOGRAPHY. 



(1) Ph. van Tieghem. Recherches sur la symetrie de structure 



des plantes vasculaires. Introduction, pp. 5-29. La Racine, 

 pp. 30-314. Annates des Sciences Naturelles, Botanique, 5 

 ser., tome xiii., 1870-71. 



(2) Van Tieghem. Memoire sur les canaux secreteurs des 



plantes. Ann. Set. Nat. Bot., 5 ser., tome xvi., 1872. 



(3) FALKENBERG. VergleicJiende Untersuchnngen fiber d. Ban d. 



der Vegctationsorgane d. Monocotyledonen. Stuttgart. 1876. 



(4) MANGIN. Origine et Insertion des racines adventives. Ann. 



Set. Nat. Bot., 6 ser., tome, xvi., 1882. 



(5) VAN TIEGHEM. Sur quelques points de l'anatomie des Cucur- 



bitacees, p. 277. Bulletin de la Societe" Botanique de Fratice, 

 tome xxix., 1882. 



(6) Morot. Recherches sur le pericycle. Ann. Sci. Nat. Bot., 1884. 



(7) Van Tieghem et Douliot. (a) Structure de la tige des 



Primeveres nouvelles du Yun-nan, p. 95. (b) Groupement 

 des Primeveres d'apres la structure de leur tige, p. 126. (c) 

 Sur les tiges a plusieurs cylindres centraux, p. 213. Bull. 

 Soc. Bot. France, tome xxxiii., 1886. 



(8) Van Tieghem et Douliot. Sur la polystelie. Ann. Sci. 



Nat. Bot., 7 ser., tome iii., 1886. 



(9) Leclerc du Sablon. Recherches sur la formation de la tige 



des Fougeres. Ann. Sci. Nat. Bot., 7 ser,, tome xi., 1890. 



(10) Van Tieghem. Traite de Botanique, 2ieme edition, 1888-91. 



