202 EVOLUTION OF PLANTS 



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 diversity 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 Sympetalte. 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 

 ca,ses be considerably increased in number. 



The development of the stamen and the pollen-sacs 



