Observations upon some Noteworthy Leaf Variations. 301 



pinnae. In a plant of moderate size the modified pinnae 

 attain a breadth of 6.5 cm., and a length of 14 cm., but this 

 latter dimension may vary very much, since it is capable of 

 extension under the circumstances already related. 



The peculiar feature of Pierson's fern is to be found in 

 the remarkable alteration affected in the pinnae, whereby the 

 frond becomes twice pinnate. In a fully modified pinna, 

 there may be as many as 50 pinnules which exactly reproduce 

 the characteristics of the pinna from which they have come, 

 even to the auricled base. (Fig. 9.) Such pinnules have a 

 maximum length of 3.2 cm., and a width of 7 mm. It is to 

 be observed, however, that the transformation is not always 

 complete, and that within the limits of individual fronds, or as 

 between different fronds, all stages of transition may be noted. 

 This is first expressed in the fact that from the same rhizome, 

 normal and modified fronds will be produced, the latter being 

 dominant and giving the prevailing character to the plant. 

 But even in the normal fronds, there is nevertheless a well 

 defined tendency to variation as expressed in those of most 

 vigorous growth, whereby the normally crenate margin be- 

 comes more deeply indented on the lower side, and the 

 leaflets become lobed or even pinnatifid. (Fig. 10.) When the 

 transformation is completed, each pinna is again completely 

 pinnate toward the apex (Fig. 9) remaining simple or pinnatifid 

 only, in the region of its base, and it is this combination of 

 features which gives to the frond as a whole, its remarkably 

 beautiful aspect. Thus I find that even in the most fully 

 developed modification, the base of each pinna is either un- 

 modified or lobed or pinnatifid for a distance of upwards of 

 1.5 cm. (Fig. 10 and n.) In most cases the auricle of the pinna 

 undergoes no alteration, but occasionally it enlarges, chiefly 

 by elongation, and assumes the aspect of a definite pinnule. 

 (Fig. 11.) In this we undoubtedly have an expression of a ten- 

 dency toward complete doubling of the organ. Wherever the 

 energy of growth is diminished, there the pinnules become 

 confluent and eventually pass by gradual stages, into the con- 

 dition of normal, simple pinnae. 



A question naturally arises in this connection. Does this 

 alteration of external form involve definite structural variations, 



