202 EVOLUTION OF PLANTS 
4 
twining, or creeping ones; or again, branches may be 
modified into thorns or tendrils. Underground por- 
tions of the stem frequently develop stolons or tubers 
which serve to propagate the plant. These are but a 
few of the manifold forms which the dicotyledonous 
stems may assume. 
The flowers of the Dicotyledons show much the same 
general structure as those of the Monocotyledons, and 
there is much the same difference between the highest 
and the lowest types, the latter hardly surpassing the 
simplest ones found among the Monocotyledons. While 
there is much variation in the number of parts in the 
flowers, it may be said that in the higher types the 
parts —at least sepals and petals, and frequently the 
stamens —are most commonly in fives. The number 
of carpels is usually smaller. 
As might be expected from the great aveyy shown 
in the flowers, there is also great variety in the char- 
acter of the fruit and seeds, much more so than is the 
case among Monocotyledons. A further discussion of 
this point, however, will be left for a later chapter. 
The carpels and stamens of the typical Dicotyledons 
resemble closely those of the Monocotyledons. The 
ovule has the same structure, but in many types has 
but a single integument, this being especially the case 
in the highest group, the Sympetale. The macrospore 
(embryo-sac) originates in the same way, and the fully 
developed gametophyte shows the egg-apparatus at the 
upper end of the sac, with the three antipodals at the 
lower end. The latter, however, may in exceptional 
cases be considerably increased in number. 
The development of the stamen and the pollen-sacs 
