INTRODUCTION. 



It has long been known from the investigations of Mirbel 1 , Link 2 , 

 Treviranus 3 and Meyen 4 and the somewhat later researches of 

 Schleiden "'. Kiitzing 6 , Unger " and Frank 8 that many hollow organs 

 such as the leaves of Allium cepa and the pith of many plants during 

 primary growth indicate the beginning of cavity -formation by the 

 separation of cells from one another, after which, as the radius of the 

 organ increases, the cavity enlarges by the death of the innermost 

 cells and the enlargement and outward movement of the more ex- 

 ternal. 



The question whether organs which thus become hollow would 

 become hollow if this first separation of cells from one another were 

 prevented, or in other words, ivhether the cells normally destroyed in 

 such cavity-formation icould die if the cells were not pulled apart by 

 surrounding tissue has never been made the subject of experiment. 

 The work here presented is in part an attempt to solve this question. 



The method pursued in the study of the subject just stated, has 

 brought results bearing also on another question which includes the 

 former and in which important conclusions have been reached by de 

 Vries and Krabbe, — the effect of radial pressure on the growth of 

 tissues in general. 



De Tries 9 in his experiments to determine the relation of nor- 

 mal cortical pressure to the formation of the annual rings of trees, 



1 Traite d'anatomie et de physiologie vegetales. Paris 1802. 



2 Grundlehren der Anatomie und Physiologie der Pflanzen. Gottingen 1807. 



3 Beitrage zur Pflanzenphysiologie. Gottingen 1811. — Physiologie der Ge- 

 wachse. 1. Bd. Bonn 1835. 



4 Xeues System der Pflanzen-Physiologie. 1. Bd. p. 313. Berlin 1837. 



5 Grundziige der wissenschaftl. Botanik. 3. Aufl. 1. Theil p. 247. Leipzig 1849. 

 f] Philosophische Botanik I. p. 316. 



7 Anatomie und Physiologie der Pflanzen p. 201. Pest, Wien und Leipzig 

 1855. 



8 Beitrage zur Pflanzenphysiologie. Leipzig 1868. 



9 De l'influence de la pression du liber sur la structure des couches ligneuses 

 annuelles. Extrait des Archives Neerlandaises 1876. Also in Flora 1876. p. 2. 



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