164 Plant Life and Evolution 



name indicates, the embryo is provided with a single 

 primary leaf, or cotyledon, and is thus easily dis- 

 tinguished from that of most dicotyledons, some 

 species of which, however, have monocotyledonous 

 embryos. 



As a rule, the monocotyledons are herbaceous 

 plants, very often having their leafy shoots arising 

 from special permanent underground structures, 

 root-stocks, bulbs, tubers, etc., and this " geoph- 

 ilous," or underground, habit of the stem has been 

 assumed to be an adaptation which accounts to some 

 extent for the reduced character of many mono- 

 cotyledons, but this is an hypothesis which requires 

 further demonstration before it can be accepted 

 without question. The leaves, as a rule, are sim- 

 ple, smooth-margined, with parallel venation, but 

 many arums and some lilies have net-veined 

 leaves, like the dicotyledons. Very few of the 

 monocotyledons are trees, the most marked excep- 

 tions being the palms and some screw-pines, al- 

 though even among the lilies there are certain 

 genera, like Yucca and Dracaena, which may attain 

 the dimensions of small trees. No monocotyledon, 

 however, shows the type of secondary thickening 

 of the trunk, which is so common in dicotyledons, 

 and in the few cases where there is a secondary 

 thickening of stem it is of quite a different char- 

 acter. 



The absence of cambium from the woody bun- 

 dles of the stem is a constant feature in mono- 



