CH.vii] THE PROTOSTELE 121 



ontogenetic advance, that a satisfactory theoretical position can be attained. 

 But even then other features must be taken into account also. The conclusions 

 derived from the study of the stele must be interpreted in harmony with the 

 results of still wider study. The biological relations of the vascular system 

 to the tissues which surround it must also be considered, and worked into a 

 conception of the general physiology of the organism : for anatomy cannot 

 stand by itself as a branch of study apart from others. Before any conclusion 

 from mere structural comparison is finally adopted it should be checked and 

 tested according to the whole body of related knowledge. This principle 

 has not always been sufficiently realised or followed by those who have placed 

 evolutionary interpretations on the facts derived from anatomical study. 



Fig. 1 14. Transverse section of a fossil Fern, Botryopteris cylindrica, 

 showing a protostele with a solid core of xylem, and peripheral 

 phloem. 



The Protostele 

 By a general consensus of opinion the non-medullated protostele has 

 been recognised as the primitive stelar type. It is actually represented in 

 all the phyla of primitive Vascular Plants, whether megaphyllous or micro- 

 phyllous. In the Filicales it is seen generally in the sporeling, and it is 

 permanently maintained in the adult stems of certain Ferns of relatively 

 primitive character. The non-medullated protostele consists of a central 

 core of xylem, often composed only of tracheides, as in Botryopteris cylindrica 

 (Fig. 114); but thin-walled parenchyma-cells may be associated with them, 



