228 HEREDITY AND EVOLUTION IN PLANTS 



Primulaceae, and Nyctaginaceae, which indicates that the 

 abnormality was not derived by inheritance from a com- 

 mon ancestor; its explanation, therefore, must be sought 

 in the influence of environment. Professor Henslow, as 

 noted above, associated the monocotyledonous tendency 

 with an aquatic habit of life, but Miss Sargant points out 

 that all the pseudo-monocots possess some underground 

 organ which is thickened as a tuber, suggesting that the signi- 

 ficant ecological factor is a geophilous, rather than aquatic, 

 habit. In further confirmation Miss Sargant notes that 

 of twenty genera having their seed-leaves fused for some 

 distance upward from the base, the majority have a 

 tuberous hyocotyl. The dicotyledonous may-apple (Po- 

 dophyllum), for example, with a geophilous habit has 

 partially united cotyledons and a stem anatomy resembling 

 that of the monocots. The only exception to correlation 

 of this nature is the mangrove (Rhizophora Mangle], 

 a tropical tree whose seeds germinate in the air while still 

 in the fruit. 



The monocotyledons are separated from the dicotyle- 

 dons by seven characters as follows i 1 



1. A single cotyledon. 



2. Stem-anatomy. 



3. Development of the embryo. 



4. Parallel venation of leaves. 



5. Short duration of primary root. 



6. Seeds with endosperm. 



7. Parts of the flower in threes. 



Of these characters, "four have been shown to appear 

 frequently among geophytes, and to be useful to the plant 

 growing under conditions which determine the geophilous 



1 As enumerated by Miss Sargant. 



