Ill] THE SEARCH FOR NEW CRITERIA 63 



material for comparison, though this is not very reliable since it varies readily 

 and often directly with the external conditions. The prothalli of Dryopteris 

 itself are found to differ greatly in size and form according to the conditions 

 under which they are grown. When crowded together they appear filamen- 

 tous, a form which is normal for some Ferns. 



XI. The prothallus bears the sexual organs: and again Dryopteris 

 offers a type common for relatively advanced Ferns (Fig. 19). Speaking 

 generally the sexual organs are more massive and more deeply sunk in the 

 prothallial tissues in relatively primitive forms, and are less massive in rela- 

 tively advanced types. Thus the structure and position of the sexual organs 

 ivill offer details of value for phyletic comparison. 



XII. The embryo-sporophyte of the Fern, which arises as the conse- 

 quence of fertilisation and is nursed for a time by the prothallus, is very 

 uniform in its structure and position in most of the Ferns of relatively recent 

 type (Fig. 25). Its parts, resulting from segmentations of the zygote which 

 correspond closely in sequence and relation to one another in the different 

 types, show striking uniformity. But this uniformity is not found among the 

 more primitive types. In some of these a suspensor is present, a feature not 

 seen in any of the most recent types of Ferns, though it is commonly present 

 in the Lycopods and in Seed-Plants. Further, the position of the first shoot, 

 which is prone in the later Ferns, is upright in some of the more primitive 

 types. Thus the embryology of the sporopJiytc offers certain features of com- 

 parative value. 



We are thus in possession of a number of characteristics of Ferns which 

 will serve as criteria for their comparison with a view to their phyletic seda- 

 tion. They are these: — 



(i) The external morphology of the shoot. 



(2) The initial constitution of the plant-body as indicated by segmentation. 



(3) The architecture and venation of the leaf. 



(4) The vascular system of the shoot. 



(5) The dermal appendages. 



(6) The position and structure of the sorus. 



(7) The indusial protections. 



(8) The characters of the sporangium, and the form and markings of 

 the spores. 



(9) The spore-output. 



(10) The morphology of the prothallus. 



(11) The position and structure of the sexual organs. 



(12) The embryology of the sporophyte. 



Before these criteria can be properly used as a basis for phyletic seriation 

 each must be considered comparatively as seen in a large number of distinct 



