Retrospective Criticism, 65 



are there called, sections) are very generally used and named : 

 the sectional names being merely intended for convenience in 

 description and reference, and not to be retained in the 

 memory. The sections are founded upon characters which 

 are considered of subordinate value to those used in distin- 

 guishing the genera : the latter depending almost solely on 

 the construction of the flower and fruit, and the former upon 

 the general habit of the plant, the form and character of the 

 leaves, stipules, &c., and such variations in the fructification 

 as are found to be but of little value in the natural order 

 to which the plant belongs. The advantage of this plan 

 appears to be, that the genera in each order, all depending 

 upon modifications of the same parts, are of nearly an equal 

 value, and the sections are only employed where there is a 

 marked difference in subordinate characters or habit between 

 the parts of a genus. Some genera will therefore contain 

 many of these sections, and others only one; in the same 

 manner that an order is sometimes found to contain only one 

 genus. Or, perhaps, it would be more correct to say, that we 

 are not yet acquainted with the other sections of the genus or 

 the other genera of the order. In illustration of this, I give 

 the characters of the genera and sections (subgenera) in the 

 tribe Thlaspideae, order Cruciferse. 



The tribe is divided into two parts : — 1. Cells of silicle 

 two- or many-seeded; 2. Cells of silicle one-seeded. 



I take the former of these, because it contains three British 

 genera : the characters are — 



Thlaspi. — Silicle emarginate at the apex, valves winged at the back ; cells 

 two- or many-seeded. 



Hutchins/«. — Silicle elliptical, valves wingless; cells two-, rarely three-, 

 seeded. 



Teesdalw. — Silicle oval, emarginate at the apex j cells two-seeded ; sta- 

 mens each with a scale on the inner side at the base. 



Platyspermum. — Silicle oblong, crowned by the short thick style; cells 

 four-five-seeded ; seeds with a broad margin. 



The genus Thlapsi is divided into the following sections 

 (subgenera) : — 



Seel. Pachyphragma. — Silicle broad. Style none. Dissepiment thick, 

 double, furnished with three longitudinal plates. Seeds four, not 

 striated. 



Sec. 2. Carpoceras. — Valves expanding at the end into a hornlike wing. 

 Dissepiment membranous, oblong. Seeds foi^r, striated. 



Sec. 3. Nomisma. — Valves with a wing along the whole back. Seeds 

 numerous, striated. 



Sec. 4. Neurotropis. — Silicle orbicular, with a narrow recess ; back of 

 Vol. VII. — No. 37. f 



