238 MORPHOLOGY 
gonia seems to be associated with the tendency to differentiate the egg 
nucleus earlier and earlier in the history of the gametophyte, and when 
this differentiation occurs before wall formation, archegonia are no 
longer possible. The compound strobili of Gnetales also, both staminate 
and ovulate, with their more or less differentiated bractlets investing 
the stamens and ovules, suggest the inflorescence of certain angiosperms. 
There is no sure record of Gnetales as fossils, and therefore all the 
evidence available indicates that they are relatively modern among 
gymnosperms. Ephedra is evidently related to the Coniferales; and 
Tumboa and Gnctum are just as evidently related to Ephedra; so that it 
is altogether probable that the Gnetales represent a somewhat modern 
offshoot from the Coniferales. 
B. ANGIOSPERMS 
General character. The angiosperms represent the culmination of 
the plant kingdom, and are plants not only of the highest rank, but also 
of the greatest importance to man. Probably they constitute also 
the greatest group of plants in the number of species, which is approxi- 
mately 125,000. When this vast assemblage of species is contrasted 
with the 450 living gymnosperms and the 4000 living pteridophytes, 
it is evident that the angiosperms form by far the largest part of our 
vascular vegetation. It is also the most modern vascular group, being 
absolutely unknown in the Paleozoic, and not very abundant until late 
in the Mesozoic. The conspicuous superficial character of the group, 
as contrasted with gymnosperms, is implied in the name, the ovule 
being enclosed by the car pel (megasporophyll) , so that the pollen grain 
does not reach the ovule, but rests on the surface of the carpel. 
The great groups. The two primary divisions of angiosperms are 
the Dicotyledons and the Monocotyledons, and the four prominent char- 
acters used in distinguishing them are as follows: (i) the embryo of a 
monocotyledon has a single terminal cotyledon and a laterally developed 
stem tip; while the embryo of a dicotyledon has a terminal stem tip 
and laterally developed cotyledons (usually two); (2) in the stem of a 
monocotyledon the vascular bundles are scattered; while in a dicoty- 
ledon they are arranged so as to form a vascular cylinder enclosing a 
pith; (3) the leaves of monocotyledons have a closed venation, that is, 
veinlets do not end freely in the margin, which is therefore entire ; 
while the leaves of dicotyledons have an open venation, veinlets ending 
freely in the margin, which is often variously toothed or lobed; (4) the 
