ON GENERA AND SPECIES. 65 



older or more perfect fronds of the plant, or different 

 plants of the same species, it is anastomose. In some 

 cases, however, the normal structure of the venation is 

 partly anastomose, and partly free, as in the genera Goni- 

 opteris, Neplirodium, Hemitelia, &c. In these genera the 

 lower or basal venules of each parcel are anastomose, 

 while the upper are free. In some genera, such as Adian- 

 tum, Lindsced, and Asplenium y the venation is normally 

 free ; anastomoses occasionally occur in different segments 

 of the same frond, but such seem more accidental than 

 normal. 



From what I have now stated it will be seen that neither 

 the form or position of the sori or venation, the absence or 

 presence of indusium, taken singly or combined, can be 

 depended on for determining natural genera. Such being 

 the case, and having had under my notice examples of 

 nearly 1,000 species of living Ferns, has led me to the con- 

 clusion that in order to form natural genera the different 

 modes of growth must be taken into account, that is the 

 manner in which the developing axis (caudex) increases, 

 its direction, and the mode of evolution and attachment of 

 the fronds. I find that Ferns present two very distinct 

 modes of growth, which are comparatively as distinct as 

 Exogens are from Endogens. My views on this subject 

 first appeared in the " Botany of H. M. S. Herald," pub- 

 lished in 1853, but as that work may not be accessible to 

 all engaged in the study of Ferns, I will repeat the sub- 

 stance of what is there given. 



If Polypodium vulgare be examined, it will be seen that 

 its fronds are produced singly from the sides of a creeping 

 progressing stem, which has its accrescent apex always in 

 advance of its young developing frond, each successive frond 

 being produced singly from special nodes formed at more or 



