BOTANY OF TERRA AUSTRALIS. 55 



great tribes. The reasons which I then assigned for this 

 arrangement appear, however, either not to have been 

 comprehended, or to have been considered too hypothetical. 

 With a view of removing the supposed obscurity and 

 strengthening my former arguments, I shall preface what 

 1 have now to say on the subject, by a few observations 

 common to both tribes. 



The natural or most common structure of Graminece is 

 to have their sexual organs surrounded by two floral enve- 

 lopes, each of which usually consists of two distinct valves : 

 but both of these envelopes are in many genera of the 

 order subject to various degrees of imperfection or even 

 suppression of their parts. 



The outer envelope or ffluma of Jussieu, in most cases, 

 containing several flowers with distinct and often distant 

 insertions on a common receptacle, can only be considered 

 as analogous to the bractese or iiivolucrum of other plants. 



The tendency to suppression in this envelope appears to 

 be greater in the exterior or lower valve, so that a gluma 

 consisting of one valve may, in all cases, be considered as 

 deprived of its outer or inferior valve. In certain genera 

 with a simple spike, as Lolium and Lepturus, this is clearly 

 proved by the structure of the terminal flower or spicula, 

 which retains the natural number of parts ; and in other 

 genera not admitting of this direct proof, the fact is estab- 

 lished by a series of species showing its gradual obliteration, 

 as in those species of Panicum which connect that genus 

 with Paspalum. 



On the other hand, in the inner envelope or calyx of 

 Jussieu, obliteration first takes place in the inner or psi 

 upper valve ; but this valve having, instead of one central 

 nerve, two nerves equidistant from its axis, I consider it as 

 composed of two confluent valves, analogous to what takes 

 place in the calyx and corolla of many irregular flowers of 

 other classes ; and this confluence may be regarded as the 

 first step towards its obliteration, which is complete in 

 many species of Panicum, in Andropogon, Pappophorum, 

 Alopecurus, Trichodium, and several other genera. 



With respect to the nature of this inner or proper enve-. 



