54 PTEROID FERNS [CH. 



difference in detail of structure, as well as in the degree of aloofness of the 

 Ferns in question indicates, that the similarity is homoplastic. In all of them 

 it is probably related very closely with increase in size of the upright soleno- 

 stelic stocks, in which no cambium provides for secondary thickening. 



The Thyrsopterideae and the Dennstaedtiinae are here regarded as relatively primitive 

 congeners of the Pteroid Ferns. All of them appear to be susceptible to increasing elabor 

 ation of their essentially solenostelic or dictyostelic structure, by medullary developments 

 These are probably to be viewed as independent consequences of adjustment to increasing 

 size, rather than as being truly homogenetic. More cogently still does this argument apply 

 to the polycyclic structure of Matonia (Vol. ll, Figs. 498, 499), for that genus is still further 

 aloof systematically, while the structural details, though similar in principle, differ in detail. 

 Still further afield systematically is the somewhat similar structure that appears in the 

 rhizome of Selaginella laevigata. For a discussion of the underlying principle of Size, 

 see Vol. I, Chapter x. What these various plants have in common is the capacity for such 

 developments as they present; and this is what gives its interest to the similar medullary 

 system to be described later in Acrosticlwni aiircitni (p. 59). 



Turning now to the development of the sorus of Pteris, it has been seen 

 that in Histioptei'is incisa the lower indusium is absent, and that the reference 

 of the receptacle to a strictly marginal origin is doubtful. This probably 

 illustrates a step in the "phyletic slide" of the sorus from the actual margin, 

 as in Lindsaya or Paesia, to the lower surface of the blade. In Pteris that 

 superficial origin of the sorus is constant, so that it has become a generic 

 character. The lower indusium is regularly absent, and the persistent upper 

 indusium corresponds now, not only in appearance but also in origin, to the 

 margin of the leaf. In fact in Pteris the slide of the originally marginal sorus 

 to the surface of the leaf has become complete. 



The history of individual development, upon which this conclusion is 

 founded, has been traced in Pteris longifolia^ serrulata, cretica, and quadri- 

 aurita: of these P. serrtdata will serve as a good example. The normal 

 marginal segmentation is seen in the young fertile pinnule running out to 

 the marginal cell itself. This continues so as to form the upper indusium, 

 which thus in appearance as also in actual ontogenetic fact is of marginal 

 origin. Meanwhile on the lower surface, and at points distinctly intra- 

 marginal, cells grow out to form the sporangia. There is no great regularity 

 in the order of their appearance, and it is soon clear that the sorus which they 

 form is of a mixed character (Fig. 619). There is no projecting receptacle 

 as in typical gradate Ferns. Nevertheless the most advanced sporangium 

 in any section taken vertically through the sorus appears about the middle 

 of the fertile area, while younger sporangia lie right and left of it (Fig. 619, E). 

 Evidence of this may often be seen in more advanced sori, in which the re- 

 ceptacle still remains flat, with a broad vascular commissure underlying it, 

 the whole being very fully covered by the marginal indusium (Fig. 619, C). 

 Still it may be noted in this section of P. cretica that the oldest sporangium, 



