THE STEM 167 



condition presented by the young plant or sporeling. Here the 

 tubular central cylinder is characterized by an internal as well as 

 an external endodermal layer. The general principle applicable to 

 this situation is that young stages frequently perpetuate conditions 

 which have disappeared in the adult. It is apparently clear from 

 commonly accepted canons of comparative anatomy which will be 

 elaborated in a subsequent chapter that leaf and seedling stem fre- 

 quently enable us to reconstruct the course of evolution in the 

 siphonostelic cylinder. 



The presence of internal phloem and endodermis has been 

 described in the former paragraph as a primitive feature of the 

 earlier types with the siphonostelic central cylinder. Other 

 important anatomical structures of lower or cryptogamic vascular 

 plants are exarch and mesarch primary wood. It will be recalled 

 that the term exarch has reference to the fact that the protoxylem 

 elements that is, the tracheary tissues which are first laid down 

 are external in position, and that the subsequent development of 

 elements of the wood is centrad, or toward the center of the organ. 

 Likewise in the case of mesarch structure the situation is first 

 characterized by the exarch condition, but after a certain progress 

 has been made the formation of new elements of the xylem shifts 

 to the external aspect of the protoxylem, and as a result we find 

 the first-formed elements more or less completely surrounded by 

 those of later origin. Later in geological time and in still higher 

 types the mesarch condition in turn gives place to the endarch 

 by the disappearance of the centrad or centripetal wood. This 

 situation can best be illustrated by a diagram. In Fig. 121 at a 

 is shown the exarch condition of development of the structures of 

 the primary wood. The small-sized elements, normally ringed or 

 spiral, are outside the later-formed scalariform or pitted tracheids. 

 In b is represented the mesarch condition, in which the development 

 is at first centrad, but later becomes peripherad, so that the pro- 

 toxylem comes to occupy a central position. In c is shown the 

 endarch condition where the seriation from the protoxylem is 

 no longer centrad but has become entirely peripherad or centrifu- 

 gal. The condition in a characterizes the organization of the 

 primary xylem in the stem of the lycopods and their allies, and is 



