340 SCIENCE PROGRESS 



manner as the pith replaced the central vascular tissue of the 

 protostele. 



Farmer and Hill (20), and Chandler (10) have expressed the 

 opinion that, in view of the difficulties inseparable from the con- 

 sideration of the pith as a morphological entity, it is better to 

 confine attention to vascular and non-vascular tissue ; or, in 

 other words, to treat the matter from the physiological rather 

 than from the morphological point of view. This opinion, it 

 may be remarked, has not, on the whole, been accepted with 

 favour ; for although physiologically it is indisputable, it 

 " radically ignores the morphological problems with which the 

 anatomist is confronted when he is endeavouring to trace out 

 the evolution of the tissues of vascular plants " (Tansley and 

 Lulham, 61). 



Thus we see that the stelar theory has undergone marked 

 changes ; indeed, there is in some quarters a return to the older 

 views of De Bary and Sachs. 



The physiological explanations for the variety of structures 

 obtaining, and questions relating to phylogeny, together with 

 certain other side issues, cannot here be dealt with ; nor, indeed, 

 is it desirable, for otherwise this introduction would become too 

 complicated, and so defeat its object. 



In conclusion, it must be borne in mind that although Van 

 Tieghem's theory is not now accepted by the majority, there is 

 no desire to underrate the great value of his work. 



A theory, after all, is nothing more than a working 

 hypothesis ; and as such, few theories have been more pro- 

 ductive than that of Van Tieghem. Without it, it is very 

 doubtful whether the enormous advance in our knowledge of 

 the structure of the Ferns during the last decade would have 

 been so great. 



Literature 



1. BELLI, Endoderma e periccilo nel G. Trifolium in rapporta colla teoria della 



stelia di V. Tieghem e Douliot, Mem. Reale Accad. Sci. Torino, 46, 1896. 



2. BOODLE, Comparative anatomy of the Hymenophyllace?e, Schizaeaceae, and 



Gleicheniaceaa. I. On the anatomy of the Hymenophyllaceae, Annals of 

 Botany, xiv. 



3. Id. II. On the anatomy of the Schizaeaceas, Ann. oj Bot. xiv. 



4. Id. III. On the anatomy of the Gleicheniaceaa, Ann. of Bot. xv. 



5. Id. IV. Further observations on Schizrea, Ann. of Bot. xvii. 



6. On descriptions of vascular structures, New Phytologist ii. 



7. Review : The anatomy of Palm roots, New Phytol. iv. 



