IN PTERIDOPHYTES 



2 I I 



The genus Selaginella includes eight species which are isophyllous, and 

 over three hundred which are anisophyllous in the vegetative region. An 

 intermediate condition is found in S. sanguinoletita, which has accordingly 

 been investigated by Goebel. 1 He found the dorsiventral structure to 

 arise under the influence of external factors, such as moisture and shade. 

 In certain strong-growing species, such as 6". caulescens, the upright axes 

 may be isophyllous, and the dorsiventrality only become pronounced in 



FIG. 106. 

 A small plant of Dp.na.ea alata. x. .r/^stipules. (After Campbell.) 



the plagiotropic upper branches. Others may be dorsiventral from the 

 first. Thus the genus illustrates gradual steps of dorsiventral development, 

 such as have been noted in Lycopodium and Vacrinium, though the details 

 are different. And the same conclusion may be drawn as in those cases, 

 viz. that the radial is the primitive form, and the dorsiventral the derivative. 



The Fern-shoot, notwithstanding the preponderance of its leaf-develop- 

 ment, may be examined from the same point of view as other Pteridophytes. 

 The ancient Marattiaceae illustrate a probable initiation of dorsiventrality 

 of the shoot within their own phylum. The young sporophyte seedling 



1 L.f. t p. 105. 



