(5 FERNS OF THE LAKE COUNTRY 



verse fissure when the spores, having reached maturity, 

 need to he dispersed, sometimes horizontal or ohlique, 

 instead of vertical. In the earlier stages of their 

 growth the sori are also covered with a thin transpa- 

 rent membrane, called an indusium. As the sori grow, 

 the indusia get broken and thrust hack, sometimes 

 flung off'. To some species there is no perceptible 

 indusium. Its presence or absence therefore affords 

 yet other means of help toward correcter classification. 



What classification itself is, how far from being 

 exact to the wonderfully arranged variety of nature, 

 however necessary it is to assist the memory and the 

 understanding of the botanist, may be seen from the 

 interpenetration and confusion of characteristics even 

 in the Table of British Ferns which follows here, and 

 may be gathered also from a few remarks by Dr. Lind- 

 ley qualifying an attempt to precisely describe only the 

 three great primary orders of Exogens, Endogens, and 

 Acrogens. Having spoken of the principal differences 

 between the three classes as to be briefly expressed 

 thus : 



EXOGENS, wood growing concentrically leaves 

 with reticulated veins flowers with their parts ar- 

 ranged in fours or fives embryo, or germ, dicotyledonous 

 (or two-leafed), 



ENDOGENS, wood confused leaves with parallel 

 veins flowers with their parts in threes germ mono- 

 cotyledonous (or one-leafed), 



ACROGENS, wood sinuous leaves forlc-veined or 

 altogether unbranchcd no flowers and no germ, 

 he adds : 



