ON GENERA AND SPECIES. 37 



up in the work. He classifies the genera under twenty- six 

 heads, which bear the name of the leading genus of each, 

 and adopts the different modes of venation and character of 

 the sori as part of his generic character, but he introduces 

 new elements in characterising his genera, such as giving 

 the form and structure of the sporangia, and the number 

 of articulations of the ring, also the different form of 

 the spores ; these form an important feature in his generic 

 as well as in his specific characters ; but with regard to 

 their value, I candidly confess that in all my observations 

 and experiences I have failed to satisfy myself that either 

 the number of articulations of the ring, or the form of the 

 spores, can in any way be practically serviceable for 

 defining either genera or species. The little value of such 

 characters become obvious on comparing his descriptions 

 of individual species with their respective generic cha- 

 racters, for example, in the generic character of Aspidium 

 the number of articulations of the ring is stated to be 

 18-20, that is, all his species of Aspidium should not have 

 less or more than the above number ; but in ten of his 

 new species of Aspidium, he describes the number of 

 articulations as much below the number assigned in the 

 character of the genus ; hence, these species according to 

 his own descriptions, should not be placed in Aspidium. 

 As far as regards the characters derived from the spores, 

 I am sensible that their forms are various and in some 

 cases peculiar ; but the spores of Ferns, like the seeds of 

 other plants, will, if gathered before they are mature, 

 shrivel and assume forms that will, in many instances, 

 appear to the microscopic observer to be normal, and 

 figures of such appearances are consequently drawn and 

 published. In my opinion much examination is still re- 

 quired, before either the structure of the sporangium or 



