LEAF-ARCHITECTURE AS ILLUMINATED BY A STUDY OF PTERIDOPHYTA. 695 



standing types of leaf-formation. Some ferns retain a very exactly equal dichotomy 

 of the primary venation even in the adult leaf. This usually goes along with a more 

 or less orbicular blade, entire or more or less laciniate, borne distally on a relatively 

 long petiole. Many genera with as a rule highly divided leaves include species 

 with such a character. This raises the question of their phyletic relation to the 

 more normal laciniate and pinnate representatives. Such characters are seen in 

 Actiniopteris radiata (fig. 27), a Fern of rather uncertain affinity. It is also 

 seen in the sterile leaf of Elaphoglossum {Rhipidopteris) peltatum, which is also 

 an isolated type (fig. 28). In both cases the venation is that characteristic of 



Fig. 27? — Basal part of the lamina of an adult 

 leaf of Actiniopteris radiata. ( x 4.) 



juvenile rather than of adult leaves. They are deeply laciniate, with a single vein 

 in each segment. They may be held to have retained and elaborated a relatively 

 primitive construction. 



A somewhat similar condition is seen in those species with entire reniform leaves, 

 which appear in several distinct genera, such as Adiantum, Gymnogramme, and 

 Scolopendrium, their outline and venation being essentially the same as in Tricho- 

 manes reniforme. A succession of the juvenile leaves of Adiantum reniforme, var. 

 asarifolium, is seen in fig. 29. The first of these shows only a slight advance upon 

 a simple dichotomy of the veins, with an entire blade (a). The further series (b-e) 

 explains itself, while the similarity of the last to the drawing (fig. 24, a) of Tricho- 

 manes punctatum is remarkable. But these Ferns are all widely apart in the system. 

 They are probably results of independent development, possibly in response to 

 similar requirements. They are perhaps best explained as extreme results of a 



