INTRODUCTION. 19 
in which the ovary has four carpels, each enclosing a single seed; and 
this, when mature fills up the cavity in such a manner, that the wall of 
the seed-vessel appears like an outer coat to the seeds, which thus do not 
seem to have any other envelope. The only true naked-seeded plants 
are the Conrrer& or Pine tribe and its allies, in which the seeds never 
are enclosed in a seed-vessel. The second Order Angiospermia, includes 
those Didynamia which have a distinct seed-capsule, usually two celled, 
each cavity containing many seeds. In the next class, TeTraDYNAMIA, 
there are also two orders, distinguished by the form of their pod-like seed- 
vessel; the first Stiiguose, having a long pod; the second Siliculose a 
short one. The Orders of the classes Monapetpaia, Diapevputa, 
Potyape.puta, depend upon the number of their stamens, and they 
have the same names as the first thirteen classes ; the number of stamens, 
however, being never less than five. The sub-division of the class 
Syneenesia (as now understood) is rather complex; and it is nearly the 
same in the Natural system. The Orders of the classes G¥NANDRIA, 
Monezcra, and Dracta, are distinguished by the number of stamens and 
are consequently Monandria, Pentandria, §c. Those of the class 
Poxyeamia are the Monecia, in which the same plant bears staminiferous, 
pistilline, and complete flowers, and the Diecia in which these occur 
on different individuals. 
NATURAL GROUPS IN THE LINNAAN SYSTEM. 
20. There are many of these orders which form groups truly natural; 
that is, which consist of genera having a large number of points of 
agreement with each other, independently of the characters in which 
the sub-division is founded. For example, one portion of the class 
Pentanpria, order Digynia corresponds with the Natural order Um- 
bellifere, (imcluding the parsley, carrot, hemlock, parsnip, &c.;) the class 
Trianpei, order Digynia, very nearly corresponds with the natural 
group of grasses, all these having three stamens and two styles, which 
combination is not found in any other plants. The Dipynamia, Gym- 
nospermia, again are the same with the Natural order Labdiate, to 
which belong the various kinds of mint, thyme, dead-nettle, &c.; and 
the class TeTRADYNAMIA corresponds with the Natural order Cruciferae, 
to which belong the mustard, cress, cabbage, turnip, stock, wall-flower, 
&c. From the predominance of the number ¢hree and its multiples in 
the parts of the flower of Endogens, we find most of this group in- 
cluded in the classes Triandria, Hexandria, and Enneandria; whilst 
the prevalence of the numbers four and five among Exogens causes 
the classes Tetrandria and Pentandria, Octandria and Decandria, with 
Icosandria and Polyandria, to contain a very large proportion of that 
division. But the Linnean system often brings together Exogens and 
