CHAPTER XVII 



COMPARATIVE ANATOMY OF ANGIOSPERMS * 



The question of the relationship of the two great divisions 

 of the Angiosperms has for many years been a matter of dis- 

 pute. Anatomically the differences between the Dicotyledons 

 and Monocotyledons are sufficiently well marked, but it has 

 not been easy to decide from ordinary anatomical data which 

 should be regarded as haying- the more primitive and antece- 

 dent organization. There can be little doubt that the two groups 

 are closely related, for in addition to the striking general re- 

 semblance of their sporophytic tissues there is almost an identi- 

 cal organization of the male and female gametophytes. The 

 Monocotyledons have bv some been regarded as primitive on 

 account of the absence of a cambium in their ordinarily closed 

 bundles. This 'view has been strengthened bv statements as to 

 their appearing earlier in the geological strata than the Dicot- 

 yledons. It is now known bevond doubt, however, that manv 

 of the earlier cryptogamous groups had well-marked secondary 

 Growth, so that the absence of cambial activity is bv no means 

 necessarily a primitive feature. Further, a more careful study 

 of plant fossris has made it clear that many of the remains for- 

 merly considered to be Monocotyledons are in reality Pterido- 

 phytes or Gymnosperms. Discussion of these interesting prob- 

 lems will be more profitable after the salient features of the 

 anatomy and development of the Angiosperms have been de- 

 scribed. 



DICOTYLEDONS 



It has been shown by Jeffrey 13 that the primitive condition 

 of the central cylinder in the Angiosperms is siphonostelic. 

 The tubular central cylinder of the seedling of Ranunculus, 



* Contributed by Professor Edward C. Jeffrey, of Harvard University. 

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